Paper - The development of the human chin (1917)
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Wallis WD. The development of the human chin. (1917) Anat. Rec. 12: 315-.
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The Development of the Human Chin
W. D. Wallis Fresno, California
Introduction
Linneus is generally given the credit of having been the first to make the observation that the chin is a characteristicallyhuman trait In none of the anatomical or anthropological discussions of the chin has the writer seen credit for this shrewd observation given to any eariier scientist. Obsen'ation and record of the fact that the chin is a characteristically human trait is, as a matter of fact, centuries older than Liimeus, and it is doubtful whether the latter did not copy it from some older authority. Pliny in Book XI, Chapter 60, of his Natural History, informs us that "no animal, with the exception of man, has either chin or cheek bones." The Greeks when in the act of supplication, touched the chin to show, as some would say, their affinity A^th the di\-ine, and, if this is true, making fitting recognition of its human peculiarity as a trait not shared by the animals. But no Greek scientist seems to have speculated about its origin.
The attempt to explain the evolution of this anatomical feature is certainly comparatively recent. It appears to be no earlier than Cuvier. Cuvier remarks that in certain quadrupeds, individuals occasionally are beneanth the upper jaw unusually small; as a result the lower jaw, by the inturning of the alveolar processes to articulate "with the unusually short upper ones, gives rise to a mental prominence bearing a close resemblance to the human chin. He claims to have seen an instance of this in a calf of Geneva, of reputed human paternity. Cuvier's explanation was a shrewd one. Sir Ray Lankester has used a precisely similar argument in accounting for the evolution of the chin of the elephant which, as he has shown, has developed pari passu as the jaw has shortened, the unsupported upper lip meanwhile lengthening into the trunk of the modem elephant.
In the past centun.^ considerable attention has been given to the development of the chin and various explanations of its evolution have been offered. Any discussion of the problem should give some re\iew of these views and we have attempted to sununarize them below.
Speech
Attention has often been called to the effect, or supposed effect of speech upon the mandibular conformation — or vice versa Osborn, for example, reminds us that the narrow passage between the alveolar processes which, in the simia, lie in almost parallel, or, in some cases, in forwardly converging lines, give small range for the action of the tongue. ^ TMiether this restriction of the play of the tongue would seriously interfere with speech, may be doubted. We speak of tongues wagging, but they do not really wig-wag so much as hump themselves while oscillating upward and do^^^lward in interfering with expiration. It is only upon failure of speech that we stick oui' tongues in our cheeks. The uplift in the human palate might be considered more favorable to speech than the increased width of the alveolar processes. Defects in the palate cause defects in speech, but we do not hear that people with long, narrow jaws have less linguistic faciUty than those with short, broad jaws.
The effect of the mandible upon speech has not, however, elicited so much defense as its converse, the effect of speech upon the mandible. In the mandible of simia will be found a deep pit lying in the inner concave surface. This seems to be absent generally in other manunals, including mankind. It accommodates the genio-glossus muscle which rises here and spreads out, fan-like, along the middle line of the lower surface of the tongue. This muscle. Dr. Louis Robinson believes' aids the tongue in sorting out the contents of the mouth. The dog, for example, seems to have considerable difficulty in getting rid of imdesired morsels, while cattle, the giraffe, and the camel, shift the undesired portions to one side, where long, projecting papillae help to work them out along the cheeks as they are agitated by the tongue. In place of this pit we find in the himian jaw a bony prominence or tubercle, and Dr. Robinson professes to be able to trace the gradual evolution of this pit for the accommodation of the genio-glossus muscle, as we find it in the simia, to the bony prominence known as the genial tubercle which replaces it in the human jaw. The pit itself is subject to much variation, being most shallow in a fossil lemur, and shallow in the anthropoid apes, because a downward tilting of the margin of the jaw below the incisor teeth gives larger surface for the attachment of the muscle. The depression seems to be least in the siajnang gibbon, while the himian jaws of Heidelberg and Naulette are said to show it to a slightly less extent than the siamang gibbon. As these are among the oldest and most simian of human jaws the correspondence is not to be lightly passed over. Klaatsch' confirms the existence of a fossa sublingualis in the Heidelberg jaw, and says that in the Hauser skull from Le Moustier no genio-glossus spinal prominence is present, its place being taken, as also in the Mauer and Krapina G. skulls, by a fossa or pit for the insertion of this muscle.^
' Men of the Old Stone Age, 100, 139-140, Scribners, 1915.
Evolution of the chin. North American Review, Sept., 1914, vol. 200, p. 438^49.
The insistence of Robinson is not without precedent. In 1904 Dr. C. Toldt, of Vienna, at a meeting of the German Anthropological Society of that city, suggested that the evolution of the chin was due to the development of the muscle emanating from the tongue and necessitated by speech. He returned to this argument the following year, in a paper entitled "Uber die Kinnknochelchen und ihre Bedeutung fur die Kinnbildung beim Menschen."^ This paper called forth a reply by von Hausemann, who insists that the ossicula mentaUa are to be otherwise accounted for, and that other causes have been primarily responsible for the evolution of the chin.^
3 Zeitschrift fur Ethnologic, 41 ('09), 554-oo5.
See also Hausemann, ib., p. 719, 721; R. R. Schmidt, Die Diluviale Vorzeit Deutschlands, 234 (Stuttgart, 1912); K. Gorjanovic-Kramberger, Der Diluviale Mensch, 176-181.
Vienna Anthropologische Gescllschaft Metteilungen, 36 ('06), 51, 54.
C. C. Blake^ considered the absence of the genial tubercles in the La Naulette jaw purely adaptive: "The relative and absolute great thickness of the jaw at its symphysis originates this shelflike structure, which is solely caused by the great deposit of osseous matter around the site of the genial tubercles."
Mr. Roosevelt finds the jaw of the chinless homo heidelbergensis so primitive that it must have made his speech thick and imperfect.^ The veteran French anthropologist, Paul Topinard, is not impressed by such arguments. In his discussion of the significance of the absence of the genial tubercles in the La Naulette jaw^ he points out that only a small portion of the muscles from the tongue, namely, the geniohyoid muscle, which reaches from the small hyoid bone at the base of the tongue to the s>Tnphysis, is attached to the lowejf part of the tubercles, the genio-glossus muscle finding attachment to the region above this. Instead of accepting the presence of tubercles as an advantage, he insists that the depression in the gorilla's mandible gives him considerable advantage over man. Albrecht found the mandible of an idiot of twenty-one years of age possessing tubercles "that attained the extraordinary eminence of 9 mm."' This outvies the tubercles of educated orators. This idiot, observes Topinard, did not have half the persuasive eloquence doubtless enjoyed by the La Naulette lady.
^ See his paper on Die Bedeutung der Ossicula Mentalia fur die Kinnbildung. Zeitsch f. Ethnol., 41 ('09), 714-721; see also P. Bartels in Int. IMonatschre f. Anat. and Phys., vol. 21, p. 179 ff.; Osborn, op. cit., 228; R. Virchow, in Zeitsch f. Ethnologie, 14 ('82), 287; Schaaffhausen, in Korr. d. Deutsch Gcsse f. Anthrop. (Jan. 1881), No. 1, p. 3; L' Anthropologic, 4 ('93), 753-754.
' Anthropological Review ('67), vol. 5, 29.5-302.
' National Geographic Magazine, Feb., 1916.
• Rev. d'Anthropologie ('86), serie 3, vol. i, 416-25.
" See Bull. Soc. Anthrop. d. Bruxelles, i ('82-'83).
Decrease in the Size of Teeth and of the Alveolar Processes
When our forebears assumed the erect posture and were able to fight freely with those fists which no longer had to serve as supports when running, the dangerous canines became shorter, and there was a gradual degeneration in the size, if not in the number also, of the teeth. The large bony alveolar processes in which they lay embedded were no longer needed, and there was a corresponding shrinkage in this bony structure. The absorption of the alveolar region leaves the lower portion of the mandible, which is more solid and less liable to change, relatively prominent and suggestive of a chin.
This view has been popular. Toldt, in the paper referred to, insists that the reduction in the size of the teeth, together with the drawing in of the enfolding alveolar processes, would tend to develop the chin. His commentator, von Hausemann^ recognizes the force of this portion of Toldt's argument. Bardeleben^^ pointed out that the building up of the chin would result from reduction in the size of the teeth, and declared he knew of no exception to this correlation. In accounting for the reduction, he not inaptly likened the building of the chin to a hillock left out-standing on a plain where erosion has reduced the general level. The osseous portion which is left outstanding, like this older stone formation, becomes a protruding chin. The receding of the lower jaw is not to be forgotten. In this recession the teeth and the alv^eolar processes are especially involved, so that the protuberantia mentalis gradually comes to the fore — as we find even with prehistoric man. Similarly, Arthur Keith, while attributing to the muscles of the tongue a tendency to give a forward development to the chin, lays more stress on the recession of the alveolar processes that accompanies reduction in the size of the teeth. ^^
Robert Munro in 1912^' attributed the prominence of the chin to retrocession of the facial bones, "as the shortening of
1' Anatomischcr Anz., 26 ('05), 107.
12 Man: A History of the Human Body, 193-194. No date.
" Paleolithic Man, 19 (Macmillan '12).
the alveolar ridges would cause the teeth to assume a more upright setting in their sockets." Walkhoff noted the tendency of the reduction in teeth and alveolar processes to give rise to a chin.i^ Weidenreich^^ viewed the progressive development of the chin as purely a passive process: it got ahead by remaining where it was, the superior alveolar region being meanwhile in retreat, Rudolf Martin is inclined to champion these views^® as is also Osborn. The latter adds to this tendency the growth and specialization of the muscles of the jaw and tongue employed in speech, though he insists that absence of the chin does not betoken inabihty to speak. Prof. T. T. Waterman has recently added his support to this school. ^^ That absorption of the alveolar processes will leave the chin prominent is proved in the changes that take place in old age, where both process and result can be observed. ^^
Thus the changes that take place during the life of the individual are to some extent an epitome of those that are recorded by prehistoric evidence. In all modern races of men the front part, of the semicircle arch of teeth has shrunk or 'withdrawn' considerably, or more than has the bony jaw in which the teeth are set. Consequently the bone projects in front of the teeth as the bony chin."^^ Robinson recognizes the argument that shrinkage of the alveolar processes gives rise to the chin, but discountenances it.
'* Die menschliche Sprache in ihrer Bedeutung fiir die funktionelle Gestalt des Unterkiefers. Anat. Anzieg., 24 ('03), 129-139; Bcitrag zur Lehre der menschlichen Kinnbildung, ib., 25 ('04) 147-160; see L'Anthropologie, vol. 15, 99-100, 235-236 ('04). Similarly Frizzi, Archiv. f. Anthrop, ('10), vol. 37, p. 255 flf.
'* Die Bildung des Kinnes und seine ungebliche Beziehung zur Sprache. Anat. Anzeig., 24 ('03-'04), 545-555.
'" Lehrbuch d. Anthropologie, 873-874, Jena, 1914.
" The evolution of the chin, .\merican Naturalist, April, 1916.
" See the description and illustration E. G. Norris, Human Anatomy, 65. Much valuable material will be found in the article by Ernst Frizzi, Untersuchungen am menschlichen Untcrkiefcr mit spezieller Beriicksichtigung der Regio mentalis. Archiv. fur Anthropologie, 37 ('10), 252-286. Contains extensive bibliography, 101 sketches, and detailed measurements of 100 mandibles of different races.
'• E. Ray Lankcster, Diversions of a Naturalist, 250-252 (London 1915).
Other Osseous Changes
The changes in the anterior portion of the mandible, if we compare that of an anthropoid with that of man, are not exhausted in the differences already noted, but include other important features. The os mentale in the simia is narrow, regular in curvature, and of smooth surface. The human os mentale often has a jagged border, and may be medially concave both in a horizontal and in a vertical plane, when viewed anteriorly. Most striking of all, it possesses, on both the right and the left side, a small bony protuberance or ossicle, which gives roughness to the contour, adds to the impression of greater breadth, and breaks up the convex lines into horizontal concave surfaces. Where normal osseous changes are not located at articular surfaces, we usually have to account for them as due, either to some shift in adjacent bony tissue which directly affects them, or to the action of muscles. The importance of the tuberosities at the s>^nphysis in gi\'ing prominence to the chin, should not be forgotten. Their importance as a factor in the development of the chin has been recognized by B. Adachi.^" Mies points out the value of the ossicula mentalia in building up the chin, and also the fact that paired muscles are associated with paired ossicles, whereas an undifferentiated muscle is associated with the undifferentiated ossicle.-^ A similar argument was later adduced by Toldt, who attempts to show that the development of the ossicula mentaUa, which proceeds along different lines in the human and in the simia, has been influential in giA^ng rise to the chin.-
Muscular and Mechanical Forces
That these bony protuberances are directly related to muscle development is more than probable. Bardeleben's extensive investigations show that man alone possess on the os mentale the paired (paariger) muscle, the musculus anomalus menti, which is associated, whether as cause or effect, with this bifurcation of the OS mentale in the himian. Not only is there this difference between simia and homo sapiens, but, as a consultation of G. Rugge's, Die Gesichtsmuskulatur der Primaten^^ will show, considerable differences in the muscular system of the two are represented in this portion of the face.
-" t^ber die Ivnochelchen in der Symphyse des Unterkiefers, Zeitsch. f. Morph. und Anthrop., ('04) 7, 369-372.
-' tlber die Knochelchen in der SjTnphyse de3 Unterkiefers vom neugeborenen Menschen. Anat. Anzeig., ('93), 8 p. 361-356.
" Die Ossicula mentalia und ihre Bedeutung fiir die Bildung des menschlichen Kinnes. Sitz. d. Kaiser, v. Akad. d. Wiss. ('05), 114, (AB. 111\ 657-92.
If the hand is held on the collar bone, the chin tilted in air, and the skin covering the chin raised as far as possible, the hand will detect movements over the surface of the clavicle. These movements are effected by a large muscle, or system of muscles, which rises in the lower lip, passes over the os mentale, down the front of the neck, and over the clavicle and some of the upper ribs. In the prosimia a large bundle of platysma pass over the OS mentale, due partly to the narrowness of the anterior margin, partly to the demand for larger muscles in the larger lips which do much more heavy work than the human lips. The tendency wherever muscles pass over a bony surface and pull against it, is to flatten that surface. Hence the regular rounded contour of the OS mentale in the simia, with absence of outstanding bony prominences such as w^e find in the human mandible.
There is, moreover, in the human, a specialization of muscular development which has proceeded far beyond that of the apes. The OS mentale is traversed also by a set of muscles known as the musculus mentalis, which run at almost right angles to the platysma. In the human face these systems are separate and specialized, while in the simia they are often so interwoven as to make it impossible to entirely separate them. In Ateles, for example, it is difficult to distinguish parts of the mentalis from the platysma, while in the chimpanzee the comphcation is even more marked. Similarly, according to Robinson, there is both greater development and greater specialization in the genioglossus muscle in human beings then is to be found in the apes. "In man the genio-glossus has become a series of a large number of independent muscular strips which are to all intents and purposes separate muscles, each with its little fiber of the hypoglossal nerve entering it in such a way as not to hamper its free movement, while in the apes it is apparently a single muscle, or a closely united group, acting en bloc."24
" Leipzig, 1887.
It has been already mentioned that the paired muscle on the OS mentale is found only in human beings, and that it seems to be related to the external genial tuberosities pecuUar to the human os mentale. Rugge noticed that the mental region of the human skull is distinguished from that of the simia by the possession of numerous tubercles which must be supposed to be the points of origin for many small muscles not to be found in the apes.
We need only consider the comparative range and facility of facial expression in the two species to grasp both the fact and the explanation of the existence of these numerous tubercles that cover the human mandible and their absence in the simia. The rapid pull of facial muscles used in articulation may have contributed not a little to this result. Some of the muscles used in such facial twitchings as laughter, for example, involves, find attachment on the anterior surface of the mandible. Here the bony prominences that rise to give these muscles attachment help in the outer construction of the chin. No one who has read the detailed account of the muscles used in facial expression, given by Prof. Arthur Thomson in his Anatomy for Art Students, can be skeptical about the much greater specialization of facial muscles in the human being and the tendency of these to elicit bony prominences on the skull and the mandible as points for attachment. ^^
It is not improbable that this difference in muscular pull will do much to explain the simian t^T^e of chin. The thick-lipped peoples, as notably the Negroes, have, of course, larger muscles to work their larger lips, and they have less prominent chins. Those negroid peoples who have thinner Hps have more prominent chins. The chin is well developed in the Eskimo, though they possess a long and heavy mandible, and large teeth. In the typical negro the chin is but feebly developed, m keeping with the heavy, thick, protruding lips, though the negro mandible is not larger, longer, nor heavier, nor are the teeth larger than those of the Eskimo. In the more orthognathous Bushman, with smaller, thinner lips, we find, on the contrary, a well developed chin of the anteriorly concave type. There is, in fact, a general, though not a complete, correlation between thick, prominent lips and retreating chins.
"An. Rep. Smith. Inst., 1914, p. 603; Knowledge (London), Nov., 1913. -' See also Alfred Fripp, Human Anatomy for Art Students.
The protruding teeth which are associated with the retreating chin, enable the animal to get rid of the food with comparative ease. In fact, if the teeth were set upright, as are ours, to disgorge would be fraught with considerable difficulty. If this advantage of being able to get rid of unwelcome and retain desired contents is to be preserved, large, long, strong upper lips are needed in a land where many hungry mouths linger round to pick up a fallen morsel or snatch a disappearing one. Our remote ancestor, 'probably arboreal,' had to keep a tight under Hp until he could carry his head as high as the descendant who became lord of all he surveyed. Then large under lips were no longer necessary. In our contemporaries they serve merely to proclaim the proximity of their simian ancestry.
Mr. G. F. Scott ElUot asserts that in consequence of the broadening of the skull and the shortening of the face into a shorter, roimded-arch shape, great strain and cross-tensions would be thrown on the extreme forward poiats of the jawbones.2« In this I do not follow him, for it seems to me the tendency would, if anything, be the opposite. Nor do I follow him in his suggestion that if the origin of bone-forming tissue may be due to muscular stresses and strains, then the production of bone-forming tissue at the chin would be favored — unless he means such stresses and strains as we have indicated. So far as mastication is concerned, the muscular stress would be much greater in the simia, which use their heavy, protruding lips in heu of free hands, to pull in their food, and certainly when eating, use them much more than we use ours. But the masticatory muscles are not attached to the chin and could scarcely affect the os mentale directly. Simia must do more severe work with canines and incisors than we do with ours, but the muscular strain comes much further back on the mandible than Elliot supposes.
"Prehistoric Man and His Story, 76-77; (London, 1915).
The muscles used in deglutition must, of necessity, be larger and stronger in the simia than in hiunan beings, and, so far as they attach to the anterior part of the mandU)le, we might expect them to exert some influence on its conformation. That these muscles do exert an influence on the shape of the chin is more than probable. Bijvoet, in a detailed study of the morphology of the musculus digestricus mandibulae in the zoological world, including man and the primates, demonstrates its varying area and method of attachment, which is not the same in those animals which chew with a scissor-like motion in the vertical plane, as in those whose jaws move with a side to side grinding motion — differences which are to some extent typified in man and the monkey, Duckworth's assurance to the contrary notwithstanding.'-^s
CHANGES IN HEAD FORM
The lower jaw is not anatomically a part of the skull, yet it would be wrong to suppose that we can consider it as a feature isolated from the remainder of the head, since, physiologically, it is an integral part of the head. The upper alveolar processes are the supplementary portions which make, with the mandible, a functional unit. The various portions of the skull are so closely interrelated, either structurally or functionally, that any considerable change in a given region is apt to be reflected by corresponding or compensating changes throughout the entire skull.29
-^ The method of muscle attachment has been well shown by C. Toldt, Dcr Winkelfortsatz des ITnterkiefers beim Menschen und bei den Saugetieren und die Beziehungen der Kaumuskeln zu demselben. Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften, 1905, vol. 114, (Abteilung 111), 315-478. Further descriptive and illustrative account of the diflferences between the human and the simian will be found in C. Toldt's papers, Der vordere Bauch des M. digastricus mandibulae und seine Varietaten beim Menschen; and, Der digastricus und die Muskeln des ]\Iundh6hlenbodens beim Orang. ; Sitzungsberichte der Kais. Acad. d. Wiss., 1907. Ab. Ill, vol. 116, p. 373-159; vol. 117 ('OS) p. 229-32-1.)
"8 Bijvoet, Zeitsch f. Morph. und Anthrop., 61 ('07-08), 24^315. W. H. L. Duckworth, Anthropology and Morphology, vol. 1 ('15).
The upper alveolar region is an incorporated portion of the facial skeleton, and is dependent for its evolution upon changes in the facial region. The face is, in turn, architecturally but a pendant portion of . the upper supporting skull case, so that changes in the skull case, affecting the points of juncture with the facial bones, facilitate changes in the facial proportions and with these the conformation of the alveolar regions. Dr. Karl Gorjanovic-Kramberger insists that an intensive construction of the chin is possible only through a change in the angle of prognathism, and reduction in length and size of teeth. The change in the angle of prognathism throws the roots of the front teeth to the fore and bony tissue must be built up there to give them strength. With this change in the angle of the teeth the mental prominence becomes more pronounced and the construction of the chin is assured.^" He alleges also, and with some show of reason, that the construction of the chin in modern man has been directly correlated with the arching of the prognathous mandible. Toldt, similarly, has attributed the formation of the chin to progressive changes accompanying the widening of the face and the arching of the mandible. ^'^
The changes that have taken place wdth the assumption of the erect posture and the use of softer food, have been fairly uniform, and are seen in many portions of the skull. As the foramen magnuni moves fonvard and the head becomes better balanced, less muscular pull is required to keep it in position. The mastoid muscles degenerate, pressure along the temporal bones decreases, the forehead emerges, and the head increases in breadth. The face, at the same time, broadens, because its foundation walls in the calvarium have begun to shift laterally. The alveolar processes must shift laterally also. This lateral spread of the palatal region arches the frontal portion of the alveolar region, which tends to be pointed or straight in the simia, and draws the incisors in, so that they no longer project, as hitherto. The face projects less, and with the straighter face the teeth, which in both simia and homo sapiens follow the facial angle, approximate the vertical; for the teeth must conform to the plane of the bony tissue in which they are set. Since the mandible is useful only as a correlative portion of the facial structure, it must conform to the changes effected in the skull. In doing so the chin becomes, of necessity, more and more prominent.
" This has been ably demonstrated by Mr. Francis Knowles in his study of the correlation between the interorbital width and other measures and indices of the human skull. Journ. of the Roy. Anth. Inst., 41 ('11), 318-49. Herman Welcker, in an article on Die Zugehorigkeit eines Unterkiefers zu einem bestimmten Schadel, etc. (Archiv. f. Anthropologie 27 ('01-02), 37-106, has ably demonstrated the interdependence of the mandible and the skull. See also Aurel Von Torok, tlber Variationen und Correlationen der Verhaltnisse am Unterkiefcr; Zeitsch f. Ethnol, 30 ('98), 125-182. The correlation is, of course, not absolute. Exceptions exist, but the reference to them as 'disharmonic' types is evidence of the rule.
" See the section on Zur Bildung des Kinnes beim Homo primigenius, in the author's Der Diluviale Mensch von Krapina in Kroatien, 171-176, (Wiesbaden, 1900).
»i Corrcsp. Blatt. d. Deutsch Gesells fur Anthrop., 35, ('04), 94; see L'Anthropologie, vol. 16, p. 583-584.
In the transition from the simian to the hmnan type, we must take into account also the change that has come about in the articulation of the incisors. In the simia the incisors meet. This is not uncommon in palaeolithic man and in some of the more prognathous people. Among ourselves it still occurs, but only in a small percentage of cases. In the gorilla, indeed, the upper canines actually overlap the lower to a marked degree, so that the diastema on the lower alveolar region is much more marked than that on the upper. .Already, then, the relative prominence of upper and lower incisors seems secured by this forward grasp of the upper canines. Man is standing proof of the triumph of the facial canines over the mandibular, for in homo sapiens all of the upper incisors, as well as the canines, easily overlap the lower, though sometimes the reverse occurs. It must, moreover, be borne in mind that the anatomical independence of the mandible, even where there is no physiological independence, allows the lower alveolar processes to acconunodate themselves more rapidly to the new conditions, and we might expect that they would be more rapidly mfluenced by changes in diet than would those of the palatal region. This would account for their more rapid retreat, while at the same time the enclosing upper incisors help by means of pressure from without.
We do, as a matter of fact, find that the changes in the mandible have gone further than those in the facial portions. In primitive man the alveolar processes are still at the outer margin of the lower facial region. But not so in the jaw of civilized man with prominent chin, where the alveolar processes lie behind the underlying heav^^^ bony tissue, so that the teeth no longer conform to the plane of the bony tissue in which they are embedded. We do not find such considerable changes in the facial portion as in the mandibular.
We conclude, then, that no one factor should be singled out and given the credit for having evolved the chin, but that many forces have contributed to this result. Widening of the dental arcade gives more play for the tongue, whether for shifting food or consonants. It makes a better masticator, and perhaps, a better lingijist. The interrelation may be close. AMiich is cause and which effect may be difficult to determine, for we have, not ■one, but many, parallel or interrelated developments converging in that peculiar human feature, the chin, which muscular forces, both within and without, have helped to design.
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