270 Anatomy of Skeleton 



more on the outer than the basal aspect : when these structures grow it is overlapped 

 by them externally and becomes a basal opening. The Digastric arises from the outer 

 surface just behind the foramen. The zygoma is not joined with the malar and does 

 not possess complete roots. The styloid process is cartilaginous. 



A posterior view shows the prominent occipital protuberance, from which fine ridges 

 radiate out into the upper part of the bone ; a partial separation of this upper part from 

 the lower may be seen at the sides, or a sagittal slit between the two halves of the supra- 

 occipital above. These slits are not always present, but when they are they run into 

 the postero-lateral and lambdoid fontanelles respectively. The occipital bone at birth 

 is in four pieces joined by cartilage. 



The junction between the bones in the new-born skull may be summed up as follows : 

 (a) Fusion of half bones across mid-line : all cartilage bones and no membrane bones. 

 (fc) Fusion of elements making single bones : has occurred in membrane bones, but not at 



all or only to some extent in cartilage bones. 



The skull grows rapidly during the first six or seven years of life : the growth 

 is largely in the facial region, the volume of which compared with that of the cranium 

 has been estimated to be i : 8 at birth and to have risen to i : 4 at five years of 

 age. The second dentition is accompanied by another period of increased develop- 

 ment, so that by the twenty-first year the ratio between face and cranium has risen 

 to i : 2. This second growth period brings the air sinuses particularly into evidence, 

 so that the expression may be considerably modified by it. 



The adult skull has a cranial capacity varying between 1,000 and 1,800 c.c., with 

 an average of about 1,400 c.c. In Europeans the skull is usually over 1,450 c.c. in 

 capacity, and such skulls are termed megacephalic. Mesocephalic skulls, such as the 

 Chinese, have a capacity between 1,350 and 1,450 c.c., while microcephalic skulls, 

 such as are found in lower races, have a capacity below 1,350 c.c.* 



The adult human skull is characterised by its relatively enormous capacity with a jaw arch 

 comparatively small. The large brain development that brings about these human characters 

 also leads to an increased width of the middle region of the skull, with shifting forward of the 

 lateral eyes and corresponding narrowing of the nose : this last condition, however, is no doubt 

 associated with the general retrogression of the facial region. The character of the human foramen 

 magnum shows complete adaptation to the erect position, the plane of the foramen being prac- 

 tically horizontal, whereas in anthropoid apes it is rather oblique and in quadrupeds it looks directly 

 backwards. The human skull is almost perfectly balanced on the vertebral column, and its muscular 

 attachments for support are proportionately weak. Other evidence of increased brain develop- 

 ment can be seen in the delayed fusion of the various elements that cover in the cranial cavity f : 

 in this connection it may be pointed out that the cranial elements have increased in relative size 

 and development, while some of the facial bones, such as the lachrymal and palate, show signs of 

 retrogression when compared with tho e seen in skulls of lower animals. 



Many of the facts mentioned above can be used as data when endeavouring to determine 

 approximately the age of a skull at the time of death. Thus the age of young skulls can be deter- 

 mined with fair accuracy from the number of erupted teeth : the presence of well-formed air sinuses 

 puts the age after puberty ; absence of fusion between basisphenoid and basioccipital places it 



* For further skull measurements the student must consult works on anthropology. 



f The separation of the bones over the vault no doubt enables the increase in size of the head to take 

 place more rapidly, but probably it is not an essential factor in the increase. The greatest proportionate 

 increase in size takes place of course during foetal life, but a more gradual growth goes on, with a marked 

 increase in weight, from the first few years of life until adult age : as old age approaches the skull decreases 

 somewhat in size and loses about two-fifths of its weight. The increase in size of the cranial bones, especially 

 after they have met at the sutures, is brought about as in the other bones by addition to the surface with 

 corresponding removal of bony tissue from their deep aspect : as growth proceeds in this way the parietal 

 and frontal eminences become less marked. 



