THE SKULL AS A WHOLE. 



THE VARIOUS FORMS OF THE SKULL. 



I. Differences according 1 to age. In the earlier stages of its development the posterior 

 part of the cranium bears a very large proportion to the anterior part ; so much so. that in 

 the second month of foetal life the line of the tentorium cerebelli is vertical to the basis 

 cranii, and divides the cranial cavity almost equally into two parts. The parietal region then 

 increases rapidly in volume, along- with the greater development of the cerebral hemispheres ; 

 the frontal region next augments ; and again, in the latter part of fcetal life, the occipital 

 region increases as the cerebrum extends backwards (Cleland). At the time of birth the 

 parietal region has reached its largest development in proportion to the occipital and frontal 

 regions. The greatest frontal breadth is then smaller in proportion to that between the 

 parietal eminences than afterwards. The base of the cranium is relatively small, and the 

 great wing of the sphenoid and the temporal squima do not extend so far upwards as subse- 

 quently is the case. The petrous, which is, however, comparatively large, is inclined more 

 forwards than in the adult. Other peculiarities, such as the existence of the fontanelles, the 

 prominence of the frontal and parietal eminences, and the absence of the mastoid processes, 

 have been referred to in describing the development of the several bones. The face at birth 

 scarcely reaches an eighth of the bulk of cranium, while in the adult it is at least a half 

 (Froriep). The skull grows rapidly during about the first seven years of life. By that time 



t 



Fig. 83. LATERAL VIEW OP THE CHILD'S SKULL AT, 

 BIRTH. (Leishman.) 



This figure shows the elongated form of the skull in 

 the infant, and the small proportion which the facial 

 hears to the cranial part, and also the lateral fontanelles 

 at the lower angles of the parietal bones. The lines indi- 

 cate the various diameters. 



certain parts, including the circumference of the 

 occipital foramen, the body of the sphenoid, the 

 cribriform plate, and the petrous division of the 

 temporal, have attained their definitive size. The 

 other regions also increase but little until the 

 approach of puberty, when a second period of 



active growth begins, affecting especially the face and the frontal portion of the cranium, 

 with which is associated the expansion of the frontal and other air-sinuses (Merkel). The 

 face becomes elongated in the progress of growth, partly by increased height of the nasal 

 fossas and adjacent air-sinuses, partly by the growth of the teeth and the enlargement of the 

 alveolar arches of the jaws. In old age the skull commonly becomes lighter and thinner, and 

 often a little smaller ; but in some cases it increases in thickness and weight owing to deposit 

 of bone on the interior of the brain-case without a corresponding degree of absorption 

 externally (Humphry). The proportion of the face to the cranium is also diminished by th.3 

 loss of the teeth and absorption of the alveolar portions of the jaws. In consequence of this 

 the upper jaw retreats, while in the lower jaw the same cause gives, especially when the mouth 

 is closed, a greater seeming prominence to the chin. (Froriep. " Characteristik des Kopfes 

 nach dem Entwickelungsgesetz desselben," 1845 ; Huschke, " Schadel, Hirn und Seele," 1854 ; 

 Virchow, " Entwickelung des Schadelgrundes," 1857 ; Humphry, "A Treatise on the Human 

 Skeleton." 1858, and "Old Age," 1889; Welcker, " Wachsthum und Bau des menschlichen 

 Schadels," 1862 ; Cleland, " On the Variations of the Human Skull," Phil. Trans., 1869 ; Merkel, 

 i( Beitrag zur Kenntniss der postembryonalen Entwicklung des menschlichen Schadels," 1882.) 



II. Sexual differences. The female skull is, in general, smaller, lighter, and smoother 

 than that of the male ; the muscular impressions are not so strongly marked, the mastoid 

 processes and the superciliary ridges are less prominent, and the frontal sinuses less developed. 

 The cranial capacity is less, on the average, by one-tenth, than that of the male in the same 

 race, and the frontal and occipital regions are less capacious in proportion to the parietal 

 (Huschke). The face is smaller in proportion to the cranium, the zygomatic arches slender, 

 and the jaws narrower and less prominent. The female skull resembles the young skull more 

 than that of the adult male ; but it must be admitted that it is often impossible to determine 

 the sex by the appearance or form of a skull. 



III. Race differences, their measurement and classification. Craniometry. The most 

 important measurements and characters in comparing skulls of different races are the cranial 

 capacity, the circumference of the cranium, the relative length, breadth and height of the 

 cranium, the degree of projection of the jaws, and the form of the nasal skeleton and the 

 orbital opening. 



