v ee 
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178 . OSTEOLOGY. 
arches). According to Froriep the mammalian occipital corresponds to the fusion of four 
vertebrae, and there is some reason for supposing that in some classes of vertebrates the 
occipital region of the primordial cranium is increased by fusion with the higher cervical 
segments. 
The form of skull characteristic of man is dependent on the large proportionate 
development of the cranial part, which contains the brain, and the reduction in size of the 
visceral part (face), which protects the organs of special sense. This leads to a decrease 
in the size and projection of the jaws, as well as a reduction in the size of the teeth. 
Associated with the smaller mandible there is a feebler musculature, with a reduced area 
of attachment to the sides of the skull. In this way the disappearance of the muscular 
crests and fossze, so characteristic of lower forms, is accounted for. At the same time the 
fact that the skull is poised on the summit of a vertical column, leads to important 
modifications in its structure. The disposition of parts is such that the occipito-vertebral 
articulation is so placed that the fore and hind parts of the head nearly balance each other, 
thus obviating the necessity for a powerful muscular and ligamentous mechanism to hold 
the head erect. 
Another noticeable feature in connexion with man’s skeleton is the prolonged period 
during which growth may occur before maturity is reached ; this is associated with a more 
complete consolidation of the skull, since bones, which in lower forms remain throughout 
life distinct, are in man fused with each other, as exemplified in the case of the presphenoid 
and postsphenoid, the occipital and the interparietal, to mention one or two instances 
among many. It is noteworthy, however, that during ontogeny the morphological signifi- 
cance of these bones is clearly demonstrated by their independent ossification. 
The points of exit of the various cranial nerves remain remarkably constant, and in 
their primitive condition serve to suggest the segmental arrangement of the cartilaginous 
chondro-cranium already referred to. Owing to the very great modifications which the 
mammalian skull has undergone in the process of its evolution, it may be pointed out that 
the passage of the nerves through the dura mater—a derivative, the readers may be 
reminded, of the primordial membranous cranium (see ante)—alone represents the primitive 
disposition of the nerves. Their subsequent escape through the bony base is a later and 
secondary development. In some cases the two, membranous or primary and the osseous 
or secondary foramina, correspond. In other instances the exit of the nerves through the 
dura mater does not coincide with the passage through the bone. 
Of interest in this connexion it may be pointed out that the foramina and canals 
which traverse the skull are either situated in the line of suture between adjacent bones 
or in the line of fusion of the constituent parts of which the bone pierced is made up. 
For example, the sphenoidal fissure is situated between the orbito and alisphenoids; the 
anterior condylic between the basi and exoccipitals ; the jugular between the petrous, 
basi, and exoccipital; the optic between the orbito-sphenoid and the presphenoid ; the 
Vidian between the alisphenoid, internal pterygoid plate, and the lingula. 
THE BONES -OF-LDEE -WPPRPERGEXTREMTRY: 
THE CLAVICLE. 
The clavicle (clavicula), or collar bone, one of the elements in the formation 
of the shoulder girdle, consists of a curved shaft, the extremities of which are 
(Oy 
fo) 
ACROMIAL 
FACET 
STERNAL _ 
FACET 
Trapezius 
major 
_ hyoid 
Pectoralis 
Sterno- 
Coxoip 
TUBERCLE 
Sterno-mastoid 
Fic. 122.—RiagHtT CLAVICLE AS SEEN FROM ABOVE. 
enlarged. The inner end, since it articulates with the sternum, is called the — 
sternal end: the outer extremity, from its union with the acromion process of the 
scapula, is known as the acromial end. 
