The doctrine of functional matrices

James Scott, M.D., D.Sc.* Belfast, Northern Ireland

I n 1955 it was suggested in a preliminary article on craniofacial an- alysis that there were nine regions within the skull which were related to the growth of different organs, each having its own particular pattern of gr0wth.l These regions were the cranial vault, the cranial base, the auditory (inner ear) capsules, the nasal region, the orbital cavities, the alveolopalatal region, the facial buttress system, muscular processes, and the pharyngeal region.

Since then Moss and co-workers29 3 independently, ha,ve carried this thesis much further in the interesting and stimulating doctrine of functional matrices. In recent papers Moss states that the head and neck consist of a number of relatively independent and yet integrated functions : digestion, respiration, speech, olfaction, audition, balance, vision, and neural integration. Each such function is carried out by a functional cranial component which consists of two parts: (1) all those “soft tissues” necessary to carry out this function, called the functional matrix, and (2) those “skeletal tissues” which serve to protect or support the functional matrix. This is called the skeletal unit.

In this article I would like to analyze certain aspects of this doctrine, with special reference to the oral cavity.

Functions of the oral cavity

The oral cavity’s functions include mastication, reception and storage of food, digestion, swallowing, speech, taste, and tongue-lip-palate tactile percep- tion. Other functions suppressed or reduced in significance in the human oral cavity are defense and offense in relation to potential enemies, the obtaining of food, paralyzing or killing prey (as in poisonous snakes), and the transpor- tation of objects, such as bones, sticks, articles of food, and offspring. In the human species many of these functions have been taken over by hands and hand-made and -used artifacts (from toothpicks to atom bombs). The one fea- ture they all share in common is neural integration, which takes place largely in the brain stem. Many but not all (for example, taste and digestion) of these functions have a skeletal component and, as Moss points out, each skeletal element may be involved in a number of functions and each function may involve a number of skeletal elements.

38

*Professor of Dental Anatomy, Queen’s University.

Volume 56 Number 1 hncti~nal matrices 39

The mandible is, in its development, growth, and range of activities, a multi- functional element of the craniofacial skeleton. The main function of the mandi- ble is masticatory. All its main elements-alveolar, basal (including the chin), muscular, and condylar-are involved. Nevertheless, each region has its own particular mode and pattern of growth. The coronoid process and the symphys- es1 region, as well as the condyle, involve at some period the conversion of cartilage into bone. Sooner or later (very much later in the case of the condyle) , this method of growth is replaced by growth by subperiosteal activity (deposi- tion and resorption of bone). Cartilage transformation is characteristic of sites of rapid depository growth. Cartilage itself is not readily absorbed, although the bone that replaces it is. An entirely cartilaginous element, such as Meckel’s car- tilage or that of the nasal capsule, is less plastic, less capable of alteration in form to meet the demands of function. Bone is a more sophisticated tissue.

The elements of the masticatory appa.ratus, a.part from the mandible, are the craniofacial buttress system of the upper facial skeleton, involving the maxil- lac, zygomatic, sphenoid, temporal, and frontal bones but not to any extent the vomer, ethmoid, inferior turbinate, or lacrimal bones. Other components are the teeth, the muscles of mastication, the mandibular joints, and the proprio- ceptive component of the nervous system. The musculature of the lips, cheeks, tongue, hyoid bone, and soft palate assists the process of mastication in a second- ary manner.

The forces used in mastication can be very great. This is reflected in the robustness of the skeletal elements and the strength of the muscles of mastica- tion, especially those used in the function of a.pproximating the lower jaw and teeth to the upper jaw and teeth.

The primary movements taking place at the mandibular joint are elevation and depression of the mandible. In many animals these are the only movements. In man protrusion, retrusion, lateral, and rotary movements are added to re- place simple chopping and piercing masticatory activities by the complex multi- directional movements used in chewing and breaking up a wide variety of foodstuffs.

In adult higher animals, mastication is the most characteristic oral function. It is not the oldest, however, which is to obtain and commence the digestion of food. This need not involve teeth or even a movable jaw joint. The elements of the reception-digestive mechanism are the lips, cheeks, tongue, and salivary glands. Elongated lips, cheek pouches, and variation in the amount and composi- tion of saliva are characteristic of certain specialized adaptations of this function.

The sense of taste aided by touch determines what food is acceptable. What is not is rejected.

Muscular function

The oral cavity is capable of expansion and contraction movements quite independent of mandibular movements. The musculature involved is that of the lips, cheeks, floor of the mouth, and soft palate. During mastication and oral digestion the oral cavity is closed in front by t)he lips and behind by the palato-

40 A%ott Am. J. Orthodontics July1969

lingual seal, which must be separated during the act of swallowing. In certain rodents the oral cavity can be divided into two compartments: (1) the anterior incisor region concerned with obtaining food and other activities and (2) the posterior molar region concerned with food storage and digestion. This capacitp- regulating function involves suckling, sucking, and blowing as well as food storage. The pressures involved, while less than those used in mastication, arc nevertheless considerable. If excessive and abnormal, they may produce deformi- ties of the dental arches, especially in the vulnerable canine-incisive region. The oral cavity also plays an important role in sound production and speech.

The swallowing of masticated and softened food (the bolus) is also a prcs- sure-exerting function in which the mylohyoid and tongue musculature exerts pressure against the hard palate. This is coordinated with elevabion of the soft palate and breaking of the palatolingual seal. Swallowing usually, and most ef- fectively, takes place with the teeth in occlusion, t,hat is, the muscles of mastica- tion cooperate with those initiating t.he act of swallowing.

The tactile function of the lips, tongue, and palate is also an ancient oral characteristic. These structures are among the earliest parts of the body to develop nerve endings and, in the adult, along with t,humbs and fingers, the most richly innervated. They have a large representation on the sensory cort,ex of the cerebral hemispheres. They play a vital part in t.he early functional role of suckling. In many animals they are the principal organs of tactile discrimina- tion and knowledge of the texture of external objects. With this form of general sensation is closely associated the specialized but similar sense of taste depending on the close contact between stimulating substance and appropriate nerve end- ings. The muscular components of this functional entity are those of the lips and tongue.

This function, along with those of oral digestion, oral hygiene, and swallow- ing, depends for effective action on the secretion of saliva from various glands which develop as outgrowths of the mouth cavity. Self-cleansing, or oral hygiene, depends on the presence of saliva and the mobility of the tongue, the tip of which can reach all parts of the mouth cavity.

The importance of the musculature

From this somewhat superficial analysis of oral function, some important generalizations can be made:

1. The oral cavity is a multifunctional anatomic unity-probably the part of the body with the greatest repertoire of functional activities, some of them vital to every multicellular organism.

2. The tissue with the greatest claim to universality of local presence is muscle, not bone. It has been shown that many of the oral functions share indi- vidual muscles and muscle groups but use them in different patterns of behavior. The various functional components of the oral cavity do not all qualify for Moss’s definition of functional cranial components, for not all of them require the presence of a skeletal component. Some, such as oral digestion, require no skeletal component at all ; others, such as tactile-taste discrimination, only in a very indirect manner (the source of attachments for muscles necessary for lip

Volume 56 Num be?- 1 Functiowal matrices 41

and tongue movements). There is, of course, a generalized skeleton of the oral cavity, made up of the mandible, maxillary, palatine, and hyoid bones, and each of these skeletal elements has a multifunctional role. Without it, such functions as mastication would be impossible, and swallowing would be difficult though not physiologically impossible. With a fixed open jaw and the appro- priate musculature, a boa constrictor is, to all intents and purposes, a swallow- ing machine! There is no mastication of the prey ; the small teeth are used only to prevent escape.

3. Nevertheless, the most universal and essential factor for #all oral functions is neural integration. Not only does every oral function require its own particu- lar group of neural reflexes, but a complex neural mechanism is also essential for coordination of the activity of the various functions. A single muscle unit, such as the mylohyoid, not only has its individual range of movements, but it is involved in a number of functional activities, such as determining the capac- ity of the oral cavity, swallowing, and mastication. Almost every function re- quires the coordinated action of numerous muscles and muscle groups.

The neural integrating system

The necessary coordination of muscle and gland activities is carried out in the brain stem but involves, in varying degrees, the activity of such regions as the cerebellum, the basal ganglia, and areas of the cerebral cortex. In the brain stem, however, the incoming sensory impluses (of general sensation and proprio- ceptive sense) from the maxillary and mandibular divisions of the fifth cranial nerve are dispersed through activation of the reticular system. so as to involve the motor and sensory nuclei of the third to the twelfth cranial nerves. Within this region there are masticatory neural connections involving the sensory nuclei of the fifth nerve aad motor nuclei of adjacent cranial nerves and also neural connection complexes in relation to swallowing, oral digestion, oral cavity capac- ity control, and every other oral function. The fundamental regulating mech- anism of all oral activities resides in the brain stem, the computer system coordinating all oral activities.

It is the neural integrating system that creates and regulates the various functional components of the oral ca.vity. It is the neural component that deter- mines the contribution of the tongue to such various functions as the tactile testing of food, swallowing, mastication, and oral hygiene. Neural connections not only determine the patterns of muscle activity but probably also their growth which, in turn, influences the development and growth of various parts of the craniofacial skeleton.4 It is the elaboration of neural patterns and connections which imposes function upon structure within the oral cavity. Without it, no matrix can be functional.

Development and growth of the oral cavity

The functional complexity of the oral cavity is reflected in the complexity of its developmental processes.

From the apparent simplicity of the primary stomatodeum, or mouth-nose cavity, an ectoderm-lined cul-de-sac, bounded by the brain capsule above, the

42 Scott Am J. Orthodontics July 1969

pericardial sac below, and the first visceral or pharyngeal arch at the sides (mandibular and maxillary components), by a well-known series of develop- mental procedures, the separate nasal and oral cavities become divided from one another. The palate dividing them contributes t.o the form and size of both the nasal and oral regions. The oral cavity in the closed position is occupied almost entirely by the tongue and the teeth ; t,he nasal cavity is occupied to a considerable degree by the nasal conchae, providing a greater area for the at- tachment of respiratory and olfactory mucous membrane. The oral cavity at rest is not an empty space but, rat.her, like the pleura,1 cavity or the resting esophagus, a potential space to be filled as required by air, fluid, or solid food.

In early fetal life its fundamental shape is already determined and depends not at all on the presence of a pre-existing bony skeleton, for there is none (only a few still widely separated centers of ossification), and only to a limited extent on its cartilaginous skeleton.

At this period the form-determining elements are the arch of the uniting palatal folds, the development of the vestibular clefts, and the size and perhaps the range of mobility of the tongue. When the bony skeleton does develop, it stabilizes and consolidates what was initiated during the preskeletal period of development. Furthermore, the skeletal period of development and growth ex- tending from about the tenth week of fetal life to adulthood can be usefully divided into an early subperiod, in which bone growth is active not only at surfaces and in regions of active cartilage proliferation but at sutures, and a late subperiod when suture growth is absent or greatly reduced. There is no well-marked demarcation between these periods, but there is a considerable overlap extending from the first months of postnatal life to the end of the first decade.

The period of most active growth, however, is preskeletal. At this time the ectoderm-covered facial processes, mandibular, maxillary, and frontonasal, con- sist of rapidly dividing multipotent, but as yet undifferentiated, cells. Growth at this stage takes place within the primary confines of the form of each process, a matter of genetically controlled multidirectional cell proliferation. With the beginning of tissue differentiation into muscle, cartilage, bone, enamel organs, gland formation, etc., growth rates and patterns become crystallized out, each tissue with its own rate and pattern of growth.

Bone growth is quite different from that of cartilage, muscle, and epithelium. The primordial matrix of the facial region as a whole consists of the primordial facial processes. Within them develop, by local differentiation or cellular migra- tion, appropriate and local skeletal elements, muscle groups, glands, blood vessels, and nerve trunks. Out of a common matrix develop a number of secondary and even tertiary matrices, each related to the development of a particular oral function and each consisting of an appropriate tissue complex. An example is the tooth germ, consisting of ectodermal enamel organ and mesodermal dental papilla and follicle. From each tooth germ develop enamel and Nasmyth’s mem- brane, dentine and pulp, cement and periodontal membrane, a complex of tis- sues which, between them, produce a tooth.

Another example is that complex organ, the tongue, composed of muscle, a

Functional matrices 43

specialized epithelium, taste buds, and serous and mucous glands. Yet another example is a typical salivary gland with its epithelial, mesodermal, vascular, and neural components. Moss’s fundamental thesis of functional matrices is, therefore, subject to further sophistication. It is an open-ended concept of great value in understanding and a stimulus to further research. His insistence on the importance of nonskeletal elements can and should be carried even further.

There is, furthermore, the matter of coordination of the growth of various functional matrices so that each has its appropriate growth space, tooth, gland, tongue, and mandibular condyle. Tooth size is related to amount of alveolar bone, upper jaw size to that of the lower jaw, tongue size t.o that of the oral cavity. If the proper size relationships break down, the result is deformity which may involve all parts of the oral cavity and its functional integrity (for example, gross acromegaly). The contributing factors, as far as we know them, are genet- ic, hormonal, nutritional, and neurologic. There is a great deal more to’ be known concerning how and when these various determinants act. We also need to study in greater depth the succession of the various stages in development of each functional matrix.

An important factor is the concept and working of the process of compensa- tion or multiple assurance. It has been demonstrated that t,he condylar cartilage is not necessary for growth and positioning of the mandible.2 If it is absent or removed, other factors, such as tooth occlusion, along with muscular growth and adaptation, can produce a mandible that is normal or almost normal in all re- spects except for the presence of its condyles. This does not mean that the con- dyles do not play a role in the growth of the mandible or in its positioning in relation to the upper jaw; rather, the condyle is only one of a number of fac- tors.

The present controversy as to whether active tooth eruption depends on pulp pressure or periodontal membrane contraction is a matter not of deciding which of these factors is responsible but of determining the role of each and of other factors in a complex process and the extent to which one factor can take over when another is put out of action. In this matter of compensation there arc wide individual variations not only between species (for example, replacement of lost organs) but within any one species (for example, healing of wounds).

Biology, unlike the classic sciences, is not a matt.er of what succeeds what in a constant sequence of events but of what available mechanisms are at work at any given stage of development. It is a multifunctional science in which ha,rdly anything is the result of a single determinant. It requires a unique attitude of mind, especially when we come to investigate its highest mystery, that of the functioning of the human mind itself.

Summary

The doctrine of functional matrices propounded by Moss is applied to the multifunctional activities of the human oral cavity. Providing it is not given too rigid a definition, as in insisting on the necessity of a skeletal component, it is suggested that this concept is of considerable value in analysis of the develop- ment, growth, and function of this important region of the body.

44 Asott Am. J. Orthodontics July 1969

REFERENCES

1. Scott, J. H.: Cranio-facial regions, D. Practitioner 5: 208-214, 1955. 2. Moss, M. L., and Rankow, R. M.: The role of the functional matrix in mandibular growth,

Angle Orthodontist 38: 95-103, 1968. 3. Moss, M. L.: The primacy of functional matrices in orofacial growth, D. Practitioner 19:

65-73, 1968.

4. Scott, J. H.: The growth and function of the muscles of mastication in relation to develop- ment of the facial skeleton and of the dentition, AX J. ORTHODONTICS 46: 429-449, 1954.