No. 2066.

APRIL 4, 1863.

A Course of Twelve LecturesON THE

STRUCTURE AND DEVELOPMENTOF THE

VERTEBRATE SKELETON.

Recently delivered at the Royal College of Surgeons of England,

BY PROFESSOR HUXLEY, F.R.S.

LECTURE IV.

MR. PRESIDENT AND GENTLEMEN,—In my last lecture I wasoccupied in the endeavour to explain to you the process of thedevelopment of the human skull. I pointed out that at firstthat skull arose by the development of the dorsal laminse uponeach side of the notochord or chorda dorsalis, and by the down-ward development of the ventral laminae. I explained in whatnianner the upper and lower laminas united so as to form upperand lower chambers, in which the great centres of the nervoussystem and the great visceral centres are contained. I thenwent on to speak of the division of the neural cavity into threevesicles, which eventually become three cerebral vesicles ; andI pointed out in what way the anterior part of the cranial cavity,that which lies in front of the middle vesicle, becomes bentdown over the anterior end of the notochord, as if it were a sortof axis or hinge, and so gives rise to the primitive cranialflexure, which I must beg you to understand is a very differentthing from that cranio-facial angle which I spoke of when dis-cussing the structure of the skull as a whole. Then I pointedout in what way the three great organs of sensation are de-veloped at the sides of the cranium-the olfactory organs, theeyes, and the auditory organs--commencing as involutions ofthe external integument, some of which, like the eye and ear,become eventually ’ shut off from the exterior, while others, likethe nose, remain constantly in communication with it. I then

went on to speak of the development of the lateral arches ofthe skull, those parts which are called visceral arches-thefirst, second, third, fourth, and so on—separated by clefts. I

pointed out in what manner, in front of these visceral arches,the anterior walls - of :the face develop ,from that great processin the middle region called the frontal nasal process. Then,leaving the ontward form of the skull in the- condition in whichit is seen in the most advanced of the diagrams (Fig. 16), I spokeof the changes taking place in the interior. In the earliest con-dition we found that the skull, like the rest of the skeleton,consisted of nothing but notochord, with the membranous sub-stance in which that notochord is contained-a substance com-

posed for the most part of indifferent tissue, and afterwardsbecoming modified into fibrous or connective tissue. I thendrew your attention to one very important circumstance-thatafter this homogeneous structure had obtained throughout thewhole length of the great centres of the spinal skeleton, in thedorsal region, the caudal region, and the cervical region, the in-different tissue became subdivided into a number of quadratemasses (Fig. 12, g), which we called the proto-vertebræ, and whichgave rise not only to the bodies of the vertebrae, but to gangliaand the dorsal layers of the muscles. It was an importantpoint for us, at least for future consideration, that this processof subdivision extends no further than the hindermost marginof the cranium, that it does not extend into the substance ofthe cranium at all; that therefore here is given, to begin with,an entire, fundamental difference between the cranium and thespinal column. Within this primitive skeleton of the skullthere is soon developed a secondary skeleton, just as within theprimitive skeleton of the spinal column there is soon developed

a secondary skeleton. The secondary skeleton of the spins.column originally consists of cartilaginous rings, which areseparate from one another, and which form the rudiments of thebodies and arches of the vertebrae. But here, again, there is afundamental difference. In the skull there is no such depositionof separate cartilaginous rings, but the anterior part of the noto-chord becomes surrounded by cartilaginous tissue, which formsa a continuous plate, and is not divided into separate masses;and in front of that point at which the notochord terminates-that is to say, immediately behind the pituitary body, imme-diately behind the cranial flexure-the plate becomes dividedinto two portions, the trabeculae cranii, then pass on each sidealong the margins of the pituitary fossa, unite in front, andthere give rise to the plate which is continued forward into thefrontal prominence of the skull. There is as complete a differ-ence in this respect between the cranium and the spine asthere is between the modes of development of the membranousskeleton in ach case. Then I spoke of the way in which theossification of the skull takes place ; and we found that in theoccipital region there is a development of an osseous centretaking place round the notochord, and forming eventually thecommencement of the basi-occipital bone, on each side ofwhich are the ex-occipitals; and at the top is another centre,which forms the supra-occipital bone, or at least that partof the occipital squama which lies beneath the superiorsemicircular line, that part which lies above this point beingdeveloped in a totally different way. Then in the side wallsof the skull you have distinct centres of ossification arisingin the cartilages which represent the ali-sphenoids; distinctcentres arising from those cartilages which represent the

orbito-sphenoids, and two distinct centres arising in the partwhich represents the basi-sphenoid, and becoming coalesced ;and seemingly two also in the pre-sphenoid ; then a single piecedeveloped in the median cartilage, which forms the ethmoidseptum. These were all the parts developed in cartilage, havingcartilaginous predecessors, if one may so say. Those boneswhich form the top or roof of the skull-that is, the upper halfof the supra.occipital, the two parietal bones, and the twofrontal bones-are developed, as I showed, in a totally differentway from membrane; they never have any cartilaginous pre-decessors at all, but are developed in membrane which is con-tinuous with,perichondrinm and periosteum. This distinctionholds good largely throughout the skull, and is one of consider-able importance, though I do not mean to say that it is alwaysdiagnostic as determining the nature of bones. Then we con-sidered the mode in which the bones about the olfactory cap-sules were developed: the two nasal bones developed frommembrane upon the upper part of the capsules; the vomer de.veloped in the middle line below, from perichondrium. Thetwo bones of Bertin are also said to be developed from peri-chondrium ; but the other bones-viz., the upper, middle, andlower spongy bones-are all developed within processes of car-tilage which precede their adult shape. The next point we tookup was a very important one-the development of the walls ofthe periotic capsules. We found that, as the olfactory capsulesbecame surrounded by cartilage, so the auditory capsules be-came surrounded by cartilage. In this cartilaginous sac threecentres appear : one in front of the anterior semicircular canal,which we called the prootic; one above, immediately in con-tact with the posterior vertical semicircular canal, which wecalled the epiotic ; and one behind, commencing immediatelybehind the fenestra rotunda, and forming eventually the floorof the tympanum and the lower part of what we know as thepetrous bone of human anatomy, and that we called the opis-thotic ossification. Not only have these ossifications definiterelations of this kind to different parts of the internal ear, butthey have also exceedingly definite relations to certain nerves,and these are of no less importance. The prootic ossificationlies immediately behind the exit of the third division of thefifth, and is perforated by the portio dura, and very commonly

376

also by the portio mollis, or proper auditory nerve; at anyrate, the great mass of it always lies behind the third divisionof the fifth : that is its landmark. The opisthotic ossifica-tion always lies immediately in front of the eighth pair, thepar vagum and those nerves associated with it, and alwaysencloses the fenestra rotunda, where that exists. Besides theseossifications which I showed in the upper part and base of theskull, there are two others, which I had not time to mention,developed in the little tongues of cartilage coming from thehinder part of the basi-sphenoidal cartilage upon each side ofthe carotids, where they enter the skull, and these are whatbecome eventually the lingulas sphenoidales, which I pointedout in the complete skull.

Besides these detailed charges in the ossification of the skull,this manner of its conversion into a complete bony box, thereare a number of exceedingly important and interesting moregeneral changes of its form which take place in the course ofdevelopment. Upon these I can only touch briefly at present,although they will interest us by-and-by, when we come tospeak of the modifications of the mammalian skull, particularlythe human skull. One of the most remarkable is the gradualdiminution of what we term the cranio-facial angle. If youtake a very young skull, and make a vertical section throughit-say a skull at the fourth month or thereabout-you findthat the basi-cranial axis and the basi-facial axis form an ex-ceedingly open angle ; but as the skull becomes older, the angles ’,become less and less-that is to say, taking the basi-cranial iline at the same inclination, the basi-facial line comea more andmore downwards, until in the higher races of mankind it maybe as little as a right angle. This is very considerably due tothe development downwards of the face, and to that downwarddevelopment not being accompanied by an equal forward de-

F1G. 25. *

Diagram of skull and face of human embryo of fourth month.a, Cranial cavity. b, Basis-cranii. c, Basis-faciei. d,Facial axis. e, Cranial axis.

velopment. Suppose these two lines (Fig. 25*, d, e) were at thesame angle, it is obvious that as the face increased in size itmust increase forwards as much as it increases downwards, andthe result would be, if you were to keep in the adult the cranio-facial angle which exists in the foetus, that you would have anexcessively prognathous skull; but in all the higher races ofmankind, at any rate, the face does not elongate forward in deve-lopment as it does downward, and the result is that this anglebecomes less and less until it reaches the adult state.

There are many important considerations connected withthis which I shall speak of more in detail by-and-by. In thesame way, if I went into the matter now, we should find thatwhat I termed, when speaking of the internal section of theskull, the tentorial angles become larger as age proceeds. Inthe fcetus the inclination of the occipital plane and of the ten-torial plane upon the basi cranial axis--the angles which theyform are much smaller than in the adult. At present, however,I cannot dwell upon these points, as I have much before me inexplaining the very remarkable changes which are undergoneby the parts of the face-that is to say, by the frontal nasalprocess, and by the visceral arches ; and the knowledge of theseis so very important in studying the relations of the humanskull with that of the lower animals, that we must examinethem with very great care.

I briefly explained in my last lecture how this frontal nasalprocess grows downwards, in a sort of tongue-shape, in themiddle line between the two nasal fossea, and throws out twoprocesses, which eventually become united with the maxil-lary process (Fig. 15, b). As this process is developed down-wards and forwards, it becomes thinner and thinner between

the nasal fossae, and in that way you have the lamina

perpendicularis and the cartilaginous internasal septum de-

veloped. Connected again at the bottom there spread out twoprocesses, partly membranous and partly cartilaginous, inwhich at an early period a deposit takes place to constitutethe pre-maxilla; so that in man, as in other mammalian ani-mals, at first the pre-maxilla is distinct from the maxilla, andthey are only subsequently united. At first the very processin which they are developed is distinct from the lateral process,and it is in consequence of the arrest of the development ofthese processes, their not uniting together, that certain formsof hair-lip are produced. Then, as I stated briefly the otherday, when the maxillary process and the median nasal processhave united together, the result is to shut off the aperturewhich places the nasal sac in communication with the internalcavity of the mouth. At present there are no posteriornares; the human skull in this state is in the condition, sofar as the median nares go, of a frog’s skull, or a lizard’s.Its conversion into the mammalian skull takes place by thedevelopment of certaia processes inwards : from the maxillaryprocess plates are thrown inwards, and they at length come tomeet in the middle line, and then they unite; the result is, ofcourse, that the median nares are now completely shut off fromthe cavity of the mouth, and the communication between theexternal nostrils and the cavity of the throat takes place throughthe posterior nostrils. It is in consequence of this mode of de-velopment that I ventured to say what I did, that the posteriornostrils are a secondary product-a structure peculiar to mam-mals and the higher reptiles, not found in the lower reptiles,and entirely arising by the throwing inwards of these processesof the palatine bone and maxilla.To understand what changes take place in the maxillary

process, we must first consider what occurs in the first visceralarch in a very early condition; and this is one of the most im-portant steps in the whole series of developmental changeswhich take place in the face. In the very young human em-bryo the two halves of the first visceral arch become unitedtogether in the middle line : thus you have the rudiment ofthe lower jaw. But, as I said on a previous occasion, a develop-ment takes place from the base of the skull in connexion withthe firstvisceral arch, and that forms what we were talking aboutjust now as the maxillary process. Very early indeed a little rod

FIG. 26.

Diagram of cephalic portion of an early human embryo.a, Anterior cerebral vesicle. b, Middle ditto. c, Thirdditto, passing into the narrow spinal portion of the nervousmass. d, Auditory sac folded inwards from the cutaneous

! layer. e, First facial (post-stomal) arch. f, Second ditto.’ g, Third ditto. h, Axis of maxillary rudiment. t, Pala-! tine. k, Pterygoid. l, Incus. m, Malleus. n, Meckel’s

cartilage, with dentary splint. o, Hyoid cartilaginous rod.q, Rudiment of large cornu of os hyoides.

of cartilage is developed within the centre of this first visceralarch (Fig. 26, e), and a little rod of cartilage is developed withinthe second one (f), and perhaps in the third (g). It has been statedby Reichert, to whom our knowledge of these facts is entirely due-his observations having been thoroughly verified by subsequentobservers,-that these cartilaginous rods are at first in con-nexion with the cartilaginous basis cranii. But if that connexionobtains, it is a very transitory one: for shortly they becomefree from the basis cranii at their inner ends, and become con-nected with the auditory capsule ; the auditory capsule is

developed by an involution of the integument; that is throwninwards, and you have a great sac formed which becomes sur-rounded by cartilage, and these two rods come into closecontact, one with the anterior part of the cartilage, and theother with the posterior part of it. Whether it is the case inman or not I cannot say, but certainly in the lower vertebratathere is a time when a little cartilaginous rod extends forwards,in connexion with the upper part of this cartilaginous rod ofthe first visceral arch, along the maxillary process, and becomes

377

connected with the under part of the olfactory capsule, or

rather with the transverse processes of cartilage constitutingthe rudiments of the lateral masses of the ethmoid (Fig. 26, 7t).If there is any cartilaginous rod in man at all developed in thisplace, it has a very short duration indeed, and very early theconnexion is entirely broken through, and there are developedin connexion with this rod two bones (Fig. 26, i, k), the anteriorone being the palatine, and the posterior the internal pterygoidor proper pterygoid. These then become connected with thebase of the skull in the manner I have explained; but at a veryearly period the connexion between the pterygoid and this car-tilaginous rod is entirely lost.Next there are some very curious changes which take place

in the cartilaginous skeleton of the first visceral arch itself.It becomes divided into three parts-the upper, middle, andlower. This upper part throws out a couple of processes (Fig.26, l, m, e); the middle part also sends down a process in thisfashion (Fig. 27, c), and this continues down into the substance

FIG. 27.

Diagram of first and second post-stomal arches of humanembryo. a, Incus. b, Malleus. c, Meckel’s cartilage.d, Dentary. e, Stapes. f, Os orbiculare. g, Upper hyoidcartilage ; between g and e is the rudiment of the stapediusmuscle. m, Middle hyoid cartilage. i, Rudiment ofcomuminor of os hyoides. &, Basi-hyal. Tympanic annulus.

of the lower jaw. In the mean time the tympanic cavity hasbecome developed in this region, and in connexion with thattympanic cavity you have the first rudiment of the tympanicbone taking its form as a ring; and I need not tell you what isthe real nature of these parts. The cartilage exists for a con-siderable time; the upper division of it you see (Fig. 27, a);it becomes ossified very early. The upper part becomes theincus; the middle becomes the malleus, the handle (b) be.coming connected with the drum of the ear; and this longpiece (Fig. 27, c) passes out from what corresponds with theGlaserian fissure-which does not exist, in the proper sense,in its early state-passes out between the petrosal and the tym-panic bone, and runs to the middle of the symphysis of thelower jaw. After this has taken place, the lower part-thiscartilage which is now called Meckel’s, on account of its dis-coverer-becomes coated, if I may so say, upon its outer sur-face by a plate of bone (Fig. 27, d), but this plate of bone iswholly independent of the cartilage-not developed in thesubstance of the cartilage, but in the soft tissue surroundingthe cartilage; and this little plate of bone formed here (d) isthe rudiment of the lower jaw. The centre of ossification (d)extends in two directions, and takes at last the well-knownshape of the lower jaw by ossification from a continuous point;and that then this (e) rapidly begins to die away, becomes ab-sorbed, and all that represents Meckel’s cartilage is the pro-cessus gracilis of the malleus, which slips into that Glaserianfissure, as we all know: then the whole of the cartilage (c)entirely vanishes. The upper part of the ossification which con-stitutes the ramus of the lower jaw comes in contact with thelower part of the squamosal bone, and forms the well-knownarticulation.That is what becomes of the lower part of the cartilaginous

skeleton of the first visceral arch. Now let us consider what

becomes of the upper part. Before doing that, we must takeinto account the metamorphosis of the cartilaginous skeleton ofthe second visceral arch. The cartilage in the second archcomes forward to meet its fellow in the middle line, as in thecase of Meckel’s cartilage; and it becomes divided into threeparts in very much the same sort of way, and the upper partaltogether disappears. (I give you now Reichert’s account.The investigation is exceedingly difficult, and there have beensome disputes about the matter, but certainly no one has dis-proved the accuracy of what he has stated, and I believe themain facts which I am about to state are correct.) The upperdivision is said to vanish. The second division is very small,and it changes its direction-comes into contact, on the onehand, with the long process of the incus, to which it becomesattached ; and, on the other hand, fits itself into the fenestraovalis (Fig. 27, e); and the communication between these twoportions of the arch (e, g), at first fibrous, eventually becomesmuscular; and I need not say that this is the stapedius muscle,which, as we all know, is inserted into this part of the stapes.The stapes, at first solid and somewhat round, eventually be-comes that stirrup-shaped bone with which we are familiar.This division of the primitive pillar of this second arch has nowbecome converted into muscle; the rest remains cartilaginousfor a very considerable time (g). The tympanic bone has nowbecome developed into the position of Fig. 27, l, and consequentlythis cartilaginous part (g) which now forms the styloid processoccupies the position which you see it to have. This portionof it (g to m) becomes converted into the stylo hyoid ligament,and that (Fig. 27, m) becomes connected with another portiondeveloped here, it is said by Reichert, from the third visceralarch, which is the body or large cornu of the hyoid (Fig. 26, g).After this cartilaginous state has existed for some time, anotherossification takes place. The part (Fig. 27, e) ossifies, and thatpart (Fig. 27, g) becomes what we know as the pyramid, whichyou are aware is fitted in between the tympanic bone and thegreat mass of the periotic capsule. Then the rest of it ossifies

gradually downwards, and in advanced age in man becomesconverted into that long styliform process which we know asthe styloid process. You will observe that this is a most ex-traordinary series of metamorphoses; and unless we compre-hend clearly this series of changes, this subdivision of the twocartilaginous arches, and their conversion into these parts, itwill be a hopeless task to identify the different parts of the jawin the lower vertebrata with those found in the higher.With that I conclude what I have to say respecting the

structure and development of the human skull. Of course I byno means pretend or dream of having given you anything likean exhaustive account of this very remarkable process; butwhat I have endeavoured to do is to enumerate the salientpoints which will be absolutely necessary for us in our coursethrough the lower modifications of the vertebrate skull. Andnow, instead of passing on directly to the rest of the humanskeleton, as one should do perhaps to be systematic, I will laythat aside, and, with these facts of the structure of the skullin man, which is somewhat complex, clearly in our minds Ihope, I will take an extreme term at the other end of the ver-tebrate series-endeavour to describe the structure of the skullin that other term-and then attempt to point out, upon gooddefinite grounds, what structures in the one correspond with thestructures we have become acquainted with in the other. HereI may say I shall have no reference to ideal notions of theskeleton, or to any conceptions of unity of plan, or definitecomposition, or the like. The arguments I shall use will all bebased upon more tangible matters than these-that is to say,upon the relations of bones to nerves, and to soft parts, and toone another; so that, whether one is right or wrong in one’sconclusions, there will be, at any rate, a positive basis whichcan be proved or disproved. That is always a great advantagein any inquiry ; it enables one to come to an end of the discus-sion in some way or other: while stopping in the region of theideal, I do not know that you ever can terminate a discussion.

(To be concluded.)

I A GELATINE E BANDAGE Foia FRACTURES.-M. Hamon,of Brussels, thinks that the starch used for fracture bandagesis too long drying, and proposes the following mixture, whichtakes only from one hour and a half to two hours to becomedry and hard. Gelatine, in small pieces, seven ounces ; waterfive ounces ; alcohol about four ounces. The latter is to beadded when the gelatine has been dissolved with a gentle heat.M. Hamon has also perforated the margins of the bandage,when cut after first application, with small holes, throughwhich the apparatus may be laced like stays.