492 



SKULL 



are marked, as in tin- whole skull-cap, by 

 the cerebral convolutions, and they contain nuttier- 

 OUH 'foramina' and 'fissures' which give passage 

 to various seta of nerves and blood-vessels. The 

 external or outer surface of the Iwse of the -kull, 

 if we consider it from before backwards, is formed 

 liy the palate processes of the superior maxillary 

 and palate bones; the vomer ; the pterygoid and 

 spinous processes of the sphenoid and part of its 

 Uxly ; the under surface of the temporal liones ; 

 utiil the occipital l>one. The most important of 

 the parts which it presents are named at fig. 4. 



The Mnrji/iology of t/ir Skull is the highest and 

 most dillieult problem of comparative anatomy, 

 and has cost the most extraordinary laliour for 

 ite solution. Goethe and Oken independently 

 suggested that the skull was to be regarded as the 

 modification of a series of four vertebra 1 , and this 

 ' vertebral theory ' was worked out in the most 

 elalmrate detail by Owen and other anatomists (see 

 SKKLETON). Huxley, however, in a celebrated 

 Crooninn Lecture (1858), revised and extended the 

 hitherto neglected embryological observations of 

 Rathke, proposed an unanswerable destructive 

 criticism of the archetypal theory, and may be said 

 to have thus definitely placed the newer view in 

 the way of general acceptance. An enormous 

 amount of detailed research, for which we are 

 indebted chiefly to Parker in England and Gegen- 

 baur in Germany, has established the newer theory 

 on the sure ground of actual observation. 



Taking first t In- simple UMOIMBted cartilaginous 

 cranium of a skate or dog-fish, with its appended 

 jaws and branchial arches, we find that in develop- 

 ment, though the notochord extends into the region 

 of the head, the vertebrae stop altogether snort 

 of it ; but that on each side of the cranium there 

 arise a pair of cartilaginous bars the trabentlai 

 or 'rafters' of the future skull and three pairs of 

 HBtflagfaMMH capsules, nasal, ocular, and auditory, 

 form round the developing sense-organs ; the nasal 

 capsules immediately unite with the ends of the 

 traliecnlsp, which are meanwhile uniting below, 

 and growing up at the sides to form the brain-case. 

 The auditory capsules become united with the 

 traheculw by the appearance of two new masses of 

 cartilage the parachordals the eyes of course 

 remaining free. At first there are no jaws, but a 

 series of seven or more similar vertical cart flagitious 

 bars or arches, considerably resembling the tral>e- 

 cuhc, between which slits open into the pharyngeal 

 cavity. The first pair of these arches develops an 

 ascending process, which passes above the develop- 

 ing mouth, and becomes the ' palato-pterygoid ' 

 arch or upper jaw, the original portion remaining 

 as the inaudible. The second pair of arches the 

 ' hvoid ' becomes more or less modified usually to 

 aid in supporting the jaws ami floor of the mouth, 

 while the remaining pairs become little modified, 

 and serve throughout life to supnort the gills. 



The more complex bony skulls of higher verte- 

 brates are now in principle readily understood. 

 The chondro-cranimii and subjacent arches in all 

 cases develop in the same wav, although reduction 

 and even atrophy of the gill arches sulisequently 

 takes place. The hones, although similar in the 

 adult, originate in two utterly distinct ways, either 

 by actual ossifications in the substance of the 

 chomlro cranium and jaws, or by the ossification of 

 overlying dermio, and are hence known as cartilage 

 IMIIM-S and membrane Ixines respectively the latter 

 corresponding to the dermal nones and teeth of 

 ganoid and elaxmohranch fishes. In mammals a 

 further extraordinary serialisation take* place: 

 the ends of the mandibular and byoid arches lose 

 their suspensory function, are taken up during 

 development into tin 1 interior of the car capmile, 

 and are metamorphosed into the auditory ossicles. 



Variotu Form* of the Skull. Age. At birlli the 

 existence of the fontanelles has already been re 

 ferred to. The frontal and parietal eminences are 

 especially prominent, and the mastoid process is 

 absent. The face is only one-eighth of the bulk 

 of the cranium, whereas in the adult the face is 

 equal to one-half. During the first seven yean- the 

 skull grows rapidly, and by this time many parts 

 have attained definite size. At the period of 

 puberty the face and regions of the air sinuses 

 undergo expansion. The face elongates owing to 

 HIM growth of teeth and the increase in the size of 

 their alveolar sockets. In old age the skull may 

 become lighter and thinner or the reverse. Loss 

 of the teeth and absorption of their sockets result 

 in diminution of the size of the face, and thus 

 the upper jaw recedes, while the chin becomes 

 prominent, 



Sex. It is not always possible to determine 

 the sex from the skull ; but, as a rule, the skull of 

 the male has more strongly marked muscular im- 

 pre-sions, while the mastoid processes, su|>creiliary 

 ridges, and air-sinuses are more pronounced than 

 in the female, whose skull generally retains the 

 leading features of a young skull. 



Race, In comparing the skulls of different races 

 of mankind it is necessary to have recourse to 

 various methods of measurement, and these are 

 usually conducted on the skulls of adult males. 

 The following is a short summary of these methods. 

 (a) Cranial Capacity. This is obtained by filling 

 the cranial cavity with shot, and then measuring 

 the quantity in a graduated vessel, special precau- 

 tions being observed in order to obtain equable 

 results. Tin- capacity of normal human crania 



Fig. 5. Typical Crania seen from the Vertex : 

 a, Negro, index 70, Dolichocephalic; !>, European, index 80, 

 Mesaticephalic ; c.Samoyed, index 86, Brachyccphalic. (After 

 Tylor.) 



varies from 60 to 110 cubic inches the average in 

 all races being 85 cubic inches e.g. Eskimo, 91'5 ; 

 European, 90-3; Chinese and Mongols, 87 "3; African 

 Negroes, 82-4; Native Australians (aboriginal), 

 79'3: Andaman Islanders, 78 - l. (6) Linear measure- 

 ment of the horizontal circumference of the cranium. 

 In the adult Euro|H*an male the average is 20'7 

 inches, and in the female 19'6 inches, (c) A third 

 method is by comparison of the relative lent/Mi, 

 breadth, and height of the cranium. The standard 

 c.f maximum length is taken as 100, and thus 

 ipox breadth _ w j Breadth or cephalic index; 



Hngth 



and on this basis skulls are classified in three 

 groups viz. : 



Brachycephallc = breadth-Index above 8ft 

 Mi-xiticeptullc = it ii from 7.1 to 80. 

 Dolichocephalic = u .. below 76. 



In a similar way the proportion of height to length 

 may l>e calculated, and a hciqht-index established. 

 It varies less than the breadth-index e.g. : 



Breadth. HH.bt 



Mongolian* of Siberia and Central Alia 83 



Andaman Inlander* 82 77 



Chinese 79 78 



KnuliKh : 76 71 



Native Auntntllans (aboriginal) 71 71 



Fiji Islanden 6 7 



