The International Journal of

Orthodontia, Oral Surgeryand Radiography

(All rights strictly reserved)

VOL. VIII ST. LOUIS, MAY, 1922

ORIGINAL ARTICLES

No.5

THE INFLUENCE OF THE FORCES OF OCCLUSION ONTHE DEVELOPMENT OF THE BONES OF THE SKULL*

By LAWRENCE W. BAKER, D.M.D.Assistant Professor of Orthodontia, Harvard Dental School, B oston, Mass.

To PROVE a biol ogic law requires much pati ent labor, and indeed even toput on a firm foundation a single physiologi c fact , as I am attempting t o

do in this series of investigations, is necessarily a long, slow process involvingmany pa instaking experiments, as well as many colla te ral observations. Asa matter of fact it was in 1909 that I began my initial experiment, and fromthat time up to th e present I have been constantly at work either experiment­ing or collecting data; and although I have brought some interesting facts tolight, still I am forced to admit I have not compassed the subject. The deeperI go into the work the more its complexities are appreciated, and I am mind­ful of the many influen ces operating both from within as well as from withoutwhich contribute in th e development of the skull. Furthermore, the relation­ship between the fun ction of the dental apparatus and the skull form in thevarious groups of lower animals as well as in the various types and races ofman is a most intricate and difficult subject to investigate. But it is th eencouragement that I am receiving from eminent anthropologists and theinterest shown by my co-workers that keep me a t t he task.

It might be added in th ese introductory r emarks, it was my intention atthis time of giving only a short account of my latest unpublished experiments,

"Read befo re th e Am eri can Societ y of Orthod onti sts. Atl an ti c Cit y. New Je"sey, April 27·30, In!.From the Research D ep artment of th e Harvard University D ental Sch ool.

I wish to express th anks fo r the in val uahle a id given by P rof . W alt er B. Cannon during theseexperiments, and fo r th e use of the Physi ological La boratory of th e Harvard Medical School, and toProf. H arvey Cushing for the use of the Surgical Laboratory. an d for help ful s ugge stio ns ; al so to P ro f.Re id Hunt for the use of h is laboratory a nd his skilled assistant.

259

260 Lawrence W. Baker

but as the editor, Dr. Dewey points out, since my last published report of thiswork, which was before the Panama-Pacific Dental Congress in 1915, manyyoung men have entered our ranks, and for the benefit of this younger elementhe has urged me to give a more complete report. I have yielded to his requestand am therefore giving this more extended account, which includes an out­line of my "Working theory, the salient facts regarding the earlier experimentsadded to my later experiments, so that the reader may trace the progress ofmy work from the beginning.

When the newborn babe enters this world, the muscles of mastication are

Fig. l.-(After Cryer.) Transverse section through the human bead In the molar region. Notethe size of the masseter muscles and the extent to which they pull Oll the outer walls of the antrumsand on the floors of the orbits.

among the first group of voluntary muscles to act. Their action means life,for it is by their function that nutrition is taken in. Long before the infantcan hold up its head, or has gained control over those useful organs, thehands, the muscles of mastication are highly developed and are used withgreat vigor.

During the act of nursing, the action of this set of muscles is so vigorousthat it demands an increased blood supply, to the extent that the heart actionis greatly increased; the excessive flow of blood to these parts is indicated bya reddening of the whole head, and the fontanelles themselves are caused to

Influences of Forces of Occlusion. 261

pulsate, so that untrained observers comment on their movement. Later,with the advent of the dental equipment, this group of muscles is given moreleverage, and its action becomes consequently more powerful ; in fact, theforce exerted on the bones of the head from the pull of these muscles duringlife is tremendous and amounts to many hundreds of thousands of tons offorce. I have long been convinced that this great force on the skull, and thegreat flow of arterial blood to the head, caused by this muscular activity, is a

Fig. 2.-(After Cryer.) Transverse section through the human head in the region of the ascend­ing rami of the lower jaw. Note the extent to which the muscles of mastication pull on the outerwalls of the nasal cavities and the close proximity of the attachment of the temporal muscles to thebrain.

powerful influence in the development of the bones of the head and the im­portant organs incased therein. It is the object of my researches to throwsome light on this important, but generally overlooked subject.

As this work is so intimately connected with the action of the musclesof mastication, it will be well to consider first a few facts pertaining to thegeneral action of this group of muscles.

These muscles to a large extent surround the cranium, extending as theydo from the temporal ridge on one side to the temporal ridge on the other side,

262 Lawrence l'V. Baker

and the internal muscles of mastication are attached surprisingly close to thebase of the brain. All this is shown in Figs. 1, 2 and 3.

It will also be observed from these illustrations that these muscles areattached to the skull, and inserted into the mandible at many different placesand in many different ways, so that they pull in as many different directions,but it is a singular fact that, varied 8S are these muscles in shape, size, powerand action, they have one eommon function, the dental equipment. The teethmay be almost termed the fulcrum of this group of muscles, because it is onlywhen the teeth are brought together forcibly, or come in contact with food inmastication, that the great power of these muscles is brought out. The truthof this statement can be tested by slowly bringing the teeth together. It

Fig. 3.-(Alter Crver.) Transverse section through the human head further back than in Fig. 2.Note-c-l , that the muscles of mastication at'tach to the base of the brain-case; 2, compare the size ofthis muscular mass to the brain itself; 3, note the outward pull 011 the nasopharynx of both the externaland internal pterygoid muscles.

will be noted that, while the mandible is swinging freely, the muscular actionis almost passive, but the instant the teeth are brought into action, the muscleson the sides of the head and face will be seen to knot out and contract withgreat vigor, and, of course, the hidden powerful internal muscles do the same.

The resultant forces of these muscles are in reality the forces of occlu­sion, with which the members of this Society are so notably familiar. We wellknow that the forces of occlusion are based on definite laws, and it was the

Influences at Forces of Ucciusum 263

early recognition of these laws by us, and our application of these laws toorthodontia, that has placed our specialty in such an eminent position.

The experiments on which this paper is based and those which I have atpresent under way, make me believe that the reaction of these occlusal forceson the skull are based on as definite laws as are the forces of occlusion them­selves; furthermore, I believe that in these reactions are locked some of thesecrets of a normal skull; for, if a group of forces acts in accordance withlaws, why should not the reactions of these forces be governed by just asdefinite laws as are the forces themselves?

. . ..' ... .., .

-

Fig. 4.-Skull of Eskimo. Observe the great facial breadth due to excessive mast icatron.Hrdlicker, Am. l\lus. of Nat. Hist.)

(Alter

Before leaving the first three illustrations, there are several other mat­ters worthy of consideration; for example, note the area of this muscular mass(including the tongue) as compared with the whole head. It will be observedthat it ranks well in size with the other structures or organs of this mostcompact, intricate and wonderful part of the human body. It is evident tome that Nature never would have devoted so much important space to thisgroup of muscles, if it were not for the benefit of the head as a whole. Ac­cording to the laws of evolution, the functional activity of such a mass of

264 Ltuorence W. Baker

muscular tissue cannot but have a direct and powerful influence in shaping thebones to which they are attached.·

Furthermore, the great supply of pure blood that the vigorous activityof these muscles demands not only rushes to these muscles, but also to otherparts of the head, and it seems no more than logical to believe that the brainitself shares in this increased blood supply. The idea of muscular activity

F ig. 5.-(After Prof. Arth ur Thornpson .)

*Figs. 4, 5 and 6, a re illus tra tio ns of the effec t of the use and di suse of the muscles of mastica­ti on on th e human sku ll. Fig. 4 is the skull of a Southam pton Eskimo (From Am. Mus. Nat. Hist. An­thropological Papers, N . Y., 1910 , Vol. V.). O bserve the great faci al breadth, with its correspondinglybroad and massive m andible. A. Hrdlicka st ates ("Contribution to the Anthropology of Central andSmith Sou nd Eskimo", Am. Mus. of Nat. Hist., Vol. V , Pt. 2 Anthropological Papers) that theirgreat development is typica l of th e Eskimo a nd is due primarily to the excess ive mastication Whichtheir tough food r equires. H . Whitney states in hi s work "Hunting with the Eskimo", that it isprobable no other race of man has a mastica tory equipment powerful enough to perfo rm the great worknecessary in the str uggle for li fe in that barren regi on . Fridtjof Nansen, in hi s " Es kim o Li fe", alsogives a very interest ing account of th e excessive u se of the teeth in their diet and in preparing tildesfo r dome st ic purposes ; an d Dr . r.:. A . H oot on , Professor of Anthropol og y, Harvard University, h aspo inted ou t to me that in the E sk imo this exc essive use of the teeth has brought about a d isharmonictype of skull form.

Fig. 5 from " A Co ns ideration of th e More Important Fact or s Concerned in the Production ofCranial F orm", by P rof. Arthur Thom pson, J o ur. Anthropological Inst., xxiii, N . S. vi, 1903, is hereintroduced to show that anatomists recognize the influence of the muscles of mastication as a fac tor indetermining skull form. The illu stration sh ow s an ingen ious experiment performed by Dr. Thompsonin wh ich he removed the top of the bra in -ca se f rom a hu man skull and cleverly inserted an elasticr ubber bla dder to wh ich were attached artificial muscles mad e of silken cord s. By inflat ing the bladder tova rious deg rees of tension, ~nd by exertin g pressu re on it by mean s of the art ific ial muscles, he wasab le to dem onstra te the vari ou s typ al skull fo rm s found in man , as may be o bse rv ed.

Fig. 6 sh ows th e effec t of lack of use. T he exceedingly poo rly develop ed mandible is causedby th e ankylosis o f th e temp or o-rn a nd ibula r articula tio n. T his unfor tu n ate de formity is typ ical o f thiscondi tio n when it OCCurs ea rly in life . ( Se e " Anthroplas ty for I ntra-A rticula r B on y and Fibrous Anky­losis of Temporo-mandibular Articulation" by John B. Murphy, M.D., LL.D., from the Jour. Am.Med. Ass n ., June 4, 1914, Vol. lxii, PI'. 1783-1794.)

Influences of Forces of Occlusion 265

and increased circulation to the adjacent parts is a well-known fact. Dr.Campbell, in his remarkable series of papers" laid great emphasis on thismatter.

It occurred to me that if my hypothesis regarding the influence of thedental equipment on the formation of the bones of the head were correct, in­terference with the laws of occlusion in the lower animals would show conse­quent effects in the formation of the bones of the skull; and if variationoccurred, it might throw some light on the most complex problem of the devel­opment of the human head.

To test this theory, the following experiment was perforrned : A litter offour rabbits was selected at the age of weaning, One of the rabbits was chlo-

Fig. 6.-Lack of growth of mandible, due toankylosis of temporo-maxillaryarticulation. (AfterJohn B. Murphy, M.D.)

Fig. 7.-Skull of a young rabbit showing thestate of development at the beginning of tbeexperiment.

roformed, and the skull procured is shown in Fig. 7. Two of the rernauunganimals were operated on by grinding down all the teeth on the right side ofthe mandible and the maxillary right central incisor. As the teeth elongated,repeated grinding rendered them useless, so that all the mastication was per­formed on the left side. The fourth rabbit was kept in the normal state forthe standard of comparison.

After seven months, the skeleton of one of the rabbits was procured andthe skull was found to vary as is shown in Fig. 8 which is a photograph of itsupper aspect. It will be noted, by the drawn lines, that there is a deviationof the bones to the left. t The suture between the parietal and frontal bones

*"Observations on Mastication", London. Lancet, July 11, 1903, "The Influence of the Contrac­tion of the masticatory muscles on the Local Circulation of Blood and Lymph."

tRight and left in this description refer to the right and left sides of the animal. (The photo­graphs are reduced in size).

266 Lawrence W. Baker

does not run strictly at right angles to the longitudinal axis of the skull; theright frontal bone projects further forward than the left one. It will also beobserved that the left zygomatic space is longer and more advanced than theright space. The most noticeable deviation is in the nasal bones, both beingtwisted to the left.

On the lower aspect of the skull (Fig. 9), it will be seen that the deviationextends throughout the entire skull." The most remarkable deviation is thatthe anterior root of the right zygomatic arch (the zygomatic process of themaxillary bone) is retreated, while the body of the right maxillary bone itself,with the teeth that it contains, is greatly advanced.

Figs. 12 and 13 show that the mandible, as might be expected, is also dis-

Fig. 8.-The upper aspect of a skull of anoperated-on rabbit. Observe the unequal devel­opment of each lateral half of the skull.

Fig. 9.-Lower aspect of Fig. 8.

torted, even to the size of the articular processes of the condyles (see Fig. 12).The one on the left, or working side, is perceptibly larger than the right.(Compare with normal mandible, Fig. 14.)

Three weeks later, autopsies were performed upon the second alteredrabbit and on the" control" animal. In dissecting out the muscles that controlthe movements of the mandible, I was struck with the unequal musculardevelopment of each lateral half of the altered animal. On the unused sidethe muscles were noticeably atrophied, and pale in color, as compared withthe working side. Later, the skulls were weighed and both the worked-on

'Contrast Figs. 8 and 9 with 10 and 11, which are the same views of the skull of the controlanimal.

Influences of Forces of Occlusion 267

skulls weighed much less than the normal skull, showing that evidently theinterference had affected the general osseous development of the head."

Figs. 15 and 16 show the upper and lower aspects of the skull of the sec­ond altered rabbit. It will be noted that it varies in a similar way to thefirst one.

Fig. 17 shows the anterior aspect of the mandibles of the animals operatedupon and the same view of the control animal. The method of interferencewith the occlusal equilibrium is plainly visible, and it is also evident that thisinterference with the equilibrium of occlusion has had its tangible influenceon the equilibrium of growth of the bones. In both mandibles a and c, of theoperated-on animals there is a striking similarity of deformity. In both casesthe ramus of the nonfunctioning side is shortened, and there is even a mod­ification in the development of the temporomandibular articulation on this

Fig. 10.-The upper aspect of the skull of anormal rabbit of the same litter. Observe thenormal development of each lateral half of theskull.

Fig. 1l.-Lower aspect of Fig. 10.

side, as indicated by the shape and size of the eondyle.] In the normal case b,it will be noted that there was an equilibrium of growth throughout, includingboth rami and condyles.

*Weight of animals: Weight of skulls:Control 1631 gms. Control animal 248 gms.1st operated on 1478 gms. 1st operated on 227 gms.2nd operated on 1735 gms. 2nd operated on 243 gms.

tThe fact that the condyles were varied by the interference with the functions of the teeth coin­cides with a theory advanced by Dr. M. H. Cryer, in his work on the "Enternal Anatomy of the Face"(pp. 157-159-First edition) regarding the skull of a human being in which 'the individual was forcedto put the mandible in an abnormal position in order to masticate. The change in the occlusal forcescaused a difference in the shape and size of the condyles. Owing to the lack of occlusion on one side,the markings of the orig'ins and insertions of the muscles on that side were much tess well definedthan on the working side. This same condition was evident in my experiment; in fact, the conditionwent so far 'that the zygomatic processes on the nonfunctioning sides were perceptibly atrophied.

268 Lawrence W. Boker

Fig. 18 shows the posterior aspect of both skulls a and c, and rev:eals thatthe growth of those bones, far remote from the teeth, has been interfered with,as indicated by the lines passing through the junction of the superior aspectof the zygomatic processes and the sides of the brain-case, as well as by a

Fig. 12.-Lower jaw of operated-on rabbit.Note the lack of development of the condyle ofthe disused side.

Fig. 13.-Lower aspect 'of Fig. 12.

Fig. 14.-Lower jaw of normal rabbit. Observe its perfect symmetry and contrast with Fig. 12.

Influences of Forces of Occlusion 269

noticeable variation in the zygomatic spaces themselves. These spaces onthe side operated upon are visibly much smaller than on the functioning side,whereas, in the normal rabbit b, there is little or no difference between them.From all points of view, the overthrow of the balance of growth here indicatedaccords with that found in the other views of the skulls, as well as in themandibles.

The results of this experiment seem remarkable to me. Who 'would havethought that, by interfering with the laws of occlusion, the skulls would havedecreased in weight, and that every suture and every bone in the head wouldhave varied as we have seen? This experiment strongly indicates howimportant is the masticatory equipment of man to the development of thehead, and it also brings fresh illustration of the importance of the sadly

Fig. lS.-Upper aspect of the skull of anotheroperated-on rabbit of the same litter. Comparewith Fig. 8 and contrast with Fig. 10.

Fig. 16.-Lower aspect of Fig. 15. Comparewith Fig. 9 and contrast with Fig. 11.

neglected deciduous dentition which serves during the important develop­mental period of childhood.

The statement that the shape of the skull in the lower animals can bemodified, by simply interfering with the function of the teeth, seems almostincredible. The truth of this statement can be better understood when westudy further into the great power of the masticatory muscular system.

In the human being, the force required to thoroughlv masticate a leancorned-beef sandwich, was estimated to amount to about three tons. Who wouldbelieve that 7,050 pounds of pressure were exerted on the bones of the skullthrough the medium of the teeth in masticating this simple article of food!

These figures were derived from the following test: A man with 32

A II (

Fig. 17.-Anterior views of mandibles . Ob ser ve similari ty of variat ion uf a and c and contrast with norm al b.

l:\:)-lo

~s~"":

""~""~~~

~

"""":

cB.rIg. Hs.-rOStertor asp ect or sxuns . U IJS('Tve srmua rrry a t varratron to r "lg. 17 .

A

influences of Forces of Occlusion

teeth, in good .cond it ion, was selected lor the test subject . The test foodconsisted of an ordinary lean corned-beef sandwich, (272 inches by 372 inches).This was masti cated until it was involuntarily swallowed. The number ofmouthfuls, and the number of chews per mouthful, and th e time consumed,were all carefully noted , the force with which the teeth were brought to­gether was estimated to be 10 pounds for each contraction of the muscles ofmastication, which is a very conservative est.imate." Th e time consumedin leisurely eating the test food was about nine minutes, and, as has beenstated, the estimated force was 7,050 pounds. A second person in making asimilar test, required 52 contract ions less than th e former, but th e meat wasmuch more tender than in the first case.

If these deductions ar e at all accurate, the for ce exerted on th e skullthrough the medium of the teeth Over a period of time can be easily calcu­lated, and will be found to be enormous. Thus we re adily see how interferingwith OIlC side of the dental equipment in an animal can affect the shape andstructure of th e skull, for , according to the laws of evolut ion, we know thatth e continuous muscular force exerted on a given bone det ermines th e shapeof that bone.

I now wish to report on my experiments with an oth er type of herbivorousanimal, th e sheep . For this experiment, tw o animals of th e same birth wereselected for the t est, and for the control, I procured their mother. In the firstanimal I eliminat ed, as best I could, the function of the left lateral half ofthe dental apparatus, by cutting off both the maxillary and mandibular teethon that side. I~ the second animal , the function of the ri ght lateral halfwas interfered with by cutting off only the mandibular teeth Oil tha t sid e.t Afterseven months, th e skull s were procured and the next series of illustrationsshows the state of growth.

Fig. 19 is frOID a ph otograph of the lower aspect of the sk ull of animalNo.1. The drawn lin e ex tending along the sutures of th e palate passesmarkedly to th e Left of the center of th e foramen magnum. Thi s shows anasymm etric growth of th e skull which cor re sponds in a stri k ing mann er tothat observed in the rabbit exp eriment, but, of course, here it is t owards th e

*Th e amount of force require d to crush Iood has been very carefully studied hy two inv est ig ators :Dr. G. V , Black and D r . Joseph Head. Dr, Black confines his work to the dir ect force r equired toc ru sh food (see Den tal Cosmos, 1895 xxxvii , 478·484) while Dr. Head directed his investigation towardsth e t ri tu ra!iou of fo od ( see Dental Cosmos, 1901i, xlviii, 1189-1D 2). Bot h of these e xp e r iments weremore to determine the cr ush ing point of food than the amou nt of force required 10 masticate Iood : so.to est imate the force requi red to mastica te a given article of food , the numbe r of closures of the jawsmu st be considered.

The foll owi ng table sh ows the comparative result s of these two investigat ions in crushing meat.It win he noted that the tr i tu ra t ion gives a lower c rushin g force, with one ex ception , than the dir ectpressur e :

D r. Bl acklhs.

30-.1,35-4060-8035-5040-(\020-2535-40

Dr. H eadlbs.

18 -228- 9

38·4 220-3510-1425 ·3016

Corned beef •.. ,., - , , , , .. , . , - , , , , .. , , .Tenderl oin of beefsteak, ve ry tender ",., , _ , .. , •.. . .R ound of beefsteak, tough " , ' " , . . , " "" ..R oast beef " .. , . . " . . , .... • .. , . . .. . . . . • . . . . . .. .. . . . . , . . •Boiled Ham . . .. . . .. .. . . . . .• . . . . . . . .. . . ... . . . . . .. .. . . .. _ .P ork C hop s •.. . . . . ... . ... . . . • , ... .. . • . . . . . . . . ...... • . .. . . . •• . . . • • .Roast Veal • . . . , , .. • . , . . ... ... .. • , " " •. . .. ", . . , . , .• .

t O wing to th e fact that a ft er the operat ion, the anima ls di d not th rive in con fine me n t , t hey we resent to the country for board , wh ere it was impossible to have them under as cl ose observation as theysho u ld have been , and , unfortu nately, during this time , the pos terior molars e rupted an d were fu nct ion­a ti ng , so that the out come of the expe riment was interfered with . Ne v er th el ess, the results are worthy ofnote.

272 Lawrence W. Baker

Fig. 19.-Lateral asymmetric development of skull of sheep due to interference with the occlusal forcesof the left lateral half of dental apparatus.

Fig. 20.-\·~erl1cal asymmetric development uf same.

Infi1lences of Fones of UcclUSWIt 273

opposite side, for in this instance the left lateral half of the dental apparatusis interfered with, while with the rabbits the right lateral half was operated on.

Not only was the growth of the skull distorted in the lateral plane, but,as shown in Fig. 20 there was also a vertical distortion as well. (The verticalmeasurements were made with long steel wire pins, one resting on correspond­ing depressions of the articular surfaces of the glenoid fOSSIe, the other pin

Fig. 21.-Vertical asymmetric development of sheep No.2. due to interference of only the lower rightteeth. (Contrast with Fig. 20.)

Fig. 22.-Section of skull No.2.

resting on the corresponding depressions of the corresponding molars. Thelatter points, I consider as accurate as could be obtained, for, as stated in thefootnote, page 273, the posterior molars erupted during the experiment andtheir function was not interfered with.)

Fig. 21 is of the skull of animal No. 2 and corresponds with Fig. 20 ofanimal No. 1. It will be remembered that in this case the interference was

274 Lawrence W. Baker

confined to the mandibular teeth of the right side and that the maxillary.teethwere not interfered with during the experiment. Here it is observed that thenonfunctionating teeth, with their alveolar process, have elongated to a markeddegree, and that the skull in the region of the temporomandibular articulationis twisted in the opposite direction, as indicated by the steel wire resting inthe glenoid fossre, This vertical twist of this part of the skull correspondswell with that in animal No.1, but, of course, in the opposite direction, becauseof the interference with the opposite side.

There is in this experiment one fact which I cannot account for, and thatis, in animal No.2 there is very little lateral twisting of the anterior part ofthe skull, while in animal No.1, it is quite marked. I now realize that itwould have been better had I interfered with both animals in the same way, asI did in my rabbit experiment.

Fig..23.-Normal growth of control sheep.

Fig. 22 is a section through the 3rd molar region of animal No.2. Hereagain, the vertical distortion is plainly visible. This view, furthermore, showsthat there has been an arrest in the development of the palate in both breadthand thickness of the right, or nonfunctionating side. This lack of growth is notconfined to the palate alone, for the breadth of the entire right lateral halfis less than that of the left, or working side. The observer can test this him­self by measuring from the nasal septum to the external surface of the bones.

Fig. 23 is of the control and shows normal development.I now present for your consideration a partial report of an experiment

on a dog.· I greatly regret thaj t.his report is not complete; the incomplete-

"Dr. Richard Landsberger of Berlin, Germany, in investigating the influence of the eruption ofthe teeth in the development of the maxillary bones and nasal cavities, removed from very young dogsone lateral half of the dental apparatus and found that not only was the growth of the maxillary bonesand nasal cavities affected, but much to his surprise, found that the removal of the teeth had brought

I njfuences of Forces o] Ucclusicn 275

Fig. 24.-Anterior view of mandible ~f dog, showing lack of development due to interference withocclusal forces of the right lower teeth.

Fig. 25.-Posterior view of Fig. 24.

Fig. 26.-0bserve the asymmetric development in the mandible of the cat, due to the removal of themandibular teeth on left side.

about an asymmetric development of the entire skull. See "Der Einfluss der Zah ne auf die Entwicklungdes Schadels" von Dr. Richard Landsberger, Berlin; ein Vortrag gehalten auf der Versarnmlung derPhysiologischen in Berlin, December 11, 1911. Arch. f. Anat. u. Physiol., 1912. Compare with my"Preliminary Study of the Influence of the Forces of Occlusion on the Development of the Bones ofthe .Skull". Read before the American Society of Orthodontists at Denver, Colo., July 13, 1910, andpublished in the Items of Lnterest, February, 1911, xxxi ii, No.2.

276 Lawrence W. Baker

FIg. 27.-Skull of same dog. Observe the vertical distortion.

Fig. 28.-Same lower aspect. Observe-the lateral twist of anterior part of skull.

influences of Forces of Occlusum 277

ness is owing to the fact that, in spite of the best of care, the entire litter,including the control, died before the prescribed termination of the experi­ment.

In this particular animal, I interfered with the funct ion of the mandibularteeth on the right side. After four months and fourteen days from the dateof the operation, the skull was obtained and the following illustration showsthe state of growth. Figs. 24 and 25 are of the mandible. It will be observedthat among other changes in the body of the bone, the condyle of the non-

Fig. 29.-0bserve the asymmetric development of the bones of the skull of a dog due to the interferencewith the function of the upper left lateral half of the dental apparatus.

working side is less well developed than that of the noninterfered-with side.(Compare with Fig. 17 rabbit experiment.)

To me it seems remarkable that there should be such a bone change inso short a time, especially as, during this period, the posterior molars eruptedand were functionating. (The condition of the animal prevented etherizationand no second interference was attempted.)

Fig. 26 is here introduced to show the effects of the complete unilateralelimination of the dental apparatus in the mandible of the cat. This specimendoes not strictly belong to this series, but it so beautifully shows the result offunctional interference that it is worthy of consideration.

278 Lawrence W. Baker

Fig. 27 is an anterior view of the dog's skull, and here a vertical distor­tion is manifest. Fig. 28 is of the lower aspect. The line drawn along theintermaxillary suture passes to the right of the foramen magnum, showingthat the anterior part of the skull had already begun to deviate.

The knowledge acquired during this preliminary experiment on the dogstood me in good stead in the next dog experiment, the results of which areshown in Figs. 29, 30 and 31.

lt will be observed in Fig. 29, which is of the lower aspect of the skull,

Fig. 30.--Upper aspect of Fig. 29.

that in this animal I quite successfully eliminated the .function of the teethof the left (right side in the photograph) lateral half of the maxillae. Theeffect of this consequent unilateral function of the forces of occlusion is clearlydemonstrated by the deformed skull. (The mandibular teeth were not experi­mented with.)

Fig. 30 shows the upper aspect of the same and it will be noted that thedeformity extends throughout the entire skull. In fact the deformity wasso excessive that during the experiment, the animal man, a most conscientiousindividual, came to me in a distressed state of mind saying, "that somethingwas wrong with the head of one of my animals, for it was becoming badly

Influences of Forces of Occlusion 279

twisted, and that one of its eyes was growing out of place." It was needlessto say that this information pleased me, for it showed that my interfer encewith the occlusal forces was' bringing about tangible results.

The effect of this unilateral interference with th e maxillary teeth hadan interesting effect on the mandibular jaw (whi ch, as before stated was not inter­fered with ) for aside from the results observed in all the other mandibles ofthe operated-on animals there was excessive atrophy, or lack of development

A

5

Fig. 3t.-Mandible of skull shown in Figs. 29 and 30. Compare the rlev elopment of the alveolarprocess in A and B. The atrophy of th e pr ocess in B is wor thy of no te a nd is due to lac k of u se, th eupper left lateral half of the den tal apparatus being removed.

of the sockets of the t eeth on the nonfuncti oning side so that these nonfune­tioning teeth were beginning to show signs of loosening. This cond it ion isillustrated in Fig. 3Ib.

There is still anoth er' gr oup of animal s that should he included in th eseresearches, and it is the primate group. As a matter of fact, I have alreadyattempted to work on these animals, but have never been able to get r esults,for they are so delicate that th ey did not long sur vive in om changeabl e NewEngland climate. It has been one of my cheri shed hopes to make a trip to

280 Laicrence W. Baker

the tropics and carryon my researches with th ese animals; but I see no pros­pects of making this extended journey, so I would therefore be grateful ifany reader from these re gions would carryon these interesting experimentsfor me. I would be glad to send them the necessary instruments and helpthem in any other way that is in my power.

In conclusion, I wish to state that, although I have been working at thisproblem for a number of years, it has progressed very slowly, due largely tothe fact that each ex pe r imen t extends over so long a p eriod of time, andalso because it is a very difficult matter to completely eliminate the func­tions from one lateral half of the dental apparatus. The persistency withwhich Nature adapts th e occlusion of the interfered-with teeth for use is re­markable, and strongly points out the importance of the dental apparatus inthe scheme of life.

I fully realize that a hostile observer could criticize this work, as far asI have carried it, as being incomplete; but those who are familiar with thebiology of bone growth, and who appreciate that muscular action, blood sup­ply, and cellular activity are all bound together in bone development, will,I think, appreciate the significance of this line of investigation, and will per­ceive its importanee in the development of the bones of the head and theorgans incase~ therein.

'I'he fact that my investi gations are, perhaps, another reason for makingthe general practi t ioner appreciate the far-reaching influence of preservingthe deciduous dentition which serves during .the important developmentalperiod of ch ildhood ; and also that orthodont ists a re r eferring to it as a pl eafo r the establishment of the oeel usal equilibrium ea rly in life, st imulate s me tocarry st ill further thi s slow and patient work.

DISCUSSION

Dr. Mila Hellman, N ew York Cily.-I would like to ask Dr. Baker a question. Ishould like to know how t he t eeth were mutilated ill the la st dog.

Dr. Baker.-The fun ction was interfered with by extractio n. In the other group of

animals, namely the rabbit and sheep, I found by working on dead animals, that the teethwere so firmly rooted, it was impossible to extract them without causing serious damage, SO

I had to resort to frequent incising of the teeth.Let me add, the animals receiv ed most excellent ca re, The various operations were

performed under an anesthetic, and the care-taker saw to it that th ey received the best of

food and shelter-in fact as Dr. Cannon said-" nowadays we treat our experimenta l animals

better than our patients."

Dr. A . L eRoy Johnson, Boston, Mass.-Dr. Baker has given another demon stration ofthe law formulated in 1884 a nd 1885 by Julius \Volff, known as Wolff ' 8 Law. It reads :

"Every change in th e form and the function of a bone or of their function alone, isfollowed by certain definite ehanges in their internal architecture, and equally definitesecondary alternations in th eir exte rnal conformation, in accordance with mathematicallaws. "

The impression one gets from Dr. Baker's demon stration is, I think, misleading. Wecannot believe, and no evidence has b een presented to sub stantiate the belief, that the formof th e human head, mor e espec ia lly th e brain case, is solely determined by the pull of the

Influences of Forces of Occlusion 281

muscles of mastication. Phylogenetically there is a correlation between the size and formof the brain case and the muscles of mastication, particularly the temporal, but a correla­tion does not necessarily designate a cause. Moreover, except under pathologic conditionsosseous tissue will not impingc upon nervous structures, the latter are physiologically domi­nant. Thus, while we recognize the action of the muscles of mastication as factors in thedevelopment of the head, it is not reasonable to give them too much emphasis. In fact, thevalidity of the universal application of 'Wolff's Law is at present being questioned. Irefer to the work of Mark Jensen on "Bone Formation."

Dr. Victor Hugo Jackson, New York City.-vVith reference to the experiments madein shutting off the air from one nasal opening, Zeim, of Germany, in his experiment s ongrowing animals, to determine the cause and effect of nasal stenosis, obstructed one nasalorifice, thus withdrawing the normal nutrition from that one contrast iug it with its fellow.

As a result, which illustrates the effect of stenosis on the development of the bones,there was observed a deviation of the intermaxillary bone and the sagittal suture towardsthe occluded side, a shortening of the nasal bone, frontal bone, horizontal plate of thepalate bone, flattening of the alveolar process, and a reduction of the distance between tile

auditory canal and the alveolar process, as well as between the zygomatic arch and thesupraorbital-border. (Jackson Orthodontia, Page 30.)

Dr. Lawrence W. Baker, Boston, (closing).-I quite agree with Dr. Jac.kson in thatshutting off the air from one nostril will bring about an asymmetric. development of thebones of the skull. Dr. Willis S. Anderson, in our country performed a similar experimentand brought about, not only a marked bony change in the skull, but also his experimentscaused, through lack of proper oxidization of the blood, many far-rcaching p/fccts on thegrowth of the animals. I strongly advise all to read the report of this classic, experimentwhich was read before the American Lnryngologieal, Rhinological and Otological Societyat Atlantic City, June, 1909.

I am glad Dr. Johnson has emphasized the fact that there are other forces whicheontribute to the development of the skull other than muscular force. No doubt I havelaid so much stress on muscular force, it may he I have unconsciously misled you. How­ever, in my introductory remarks I stated there are two great groups of forces at workwhich combine to determine skull form-one group operating from within, while the othergroup operates from without. However, I think all 'will agree with me in that muscularforce is a most important factor in the latter group, and that the bony changes were broughtabout primarily by my interfering with the occlusal forces, which after all are in realitynothing more or less than the reaction of the forces of the muscles of mastication.

Dr. J. Lou:e Young, New F ork City.-Take one of these puppy dogs and feed him onmilk. As long as you keep him alive he does not chew auything, 311(1 see what you get.Do not touch his teeth.

Dr. Baker.-What do you think you wiII getf

Dr. Young.-I don't know anything about it. (Laughter.) I want you to find out.