YAEIOUS FOEMS OF THE SKULL. 



77 



cnd*stapes (according to Huxley and Parker) : and 5th, the lower or thyi-o-hyoid ; 

 following which, 6th, &c., are the branchial plates, which, though never attaining- 

 even the cartilaginous condition in the foetus of man and the higher animals, 

 yet give indications of the tendency to the continuation backwards of \dsceral 

 arches in the region between the head and the tnmk. 



The origin of the walls of the cranial cavity from the same embrj'onic elements 

 as the vei-tebrffi, and the prolongation of the notochord of the embryo into the 

 substance of the basioccipital and part of the basisphenoid bones, are of themselves 

 strong arguments in suiiport of the central character of these two osseoiis elements. 

 But. on the other hand, it is to be remarked that the notochord does not reach the 

 region of the sella tui-cica. but is directed towards the dorsum sell^e, and thus 

 leaves a pai-t of the basisphenoid and the whole of the presphenoid and meseth- 

 moid unoccupied by any tme central or vei-tebral axis. And according to Parker's 

 views the last-named parts, arising in the trabeculaj cranii. are originally double, 

 and only secondarily become mesial and single. Fm-ther. it is contended that there 

 is not. as occurs throughout the whole of the vertebral column, any primary divi- 

 sion of the fonnative cartilage marking out foiu- or even two of these centres of 

 the basis of the skull, and it is only after ossification has begun separately in 

 each that the divisions between them become apparent. 



The similaiity of constraction, then, implied in the vertebrate theory of the 

 skull belongs chiefly to the region of the occipital bone ; and while it may be 

 admitted that in other parts of the skull further foi-ward there are cu-cumstances 

 of correspondence with the vertebrate plan, yet in these there are also differences, 

 and we must therefore await the fuller and more certain detennination of several 

 homological points, at present involved in doubt, before any general theory of 

 cranial moi-phology worthy of general adoption can be framed. (See especially 

 W. K. Parker' s paper on the " Stracture and Development of the Skull in the 

 Pig," Phil. Trans., 187."? ; and in addition to the works previously refen-ed to, 

 Cleland, "On the Relations of the Vomer.'' &c., Ti-ans. Roy. Soc, 1862': Spix. 

 " Cephalogenesis," 1815 ; Hallman, " Die Vergleich. Anat. des Schlafenbeins," 1837. 



THE VARIOUS FORMS OF THE SKXTLIi, 



I. Differences according- 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 di\'ides the cranial ca^-ity almost 



Fi;r. 65. 



Fig. 65. — Lateral View of the 

 Child's Head at Birth (from 

 Leishman). 4 



This figure shows the peculiarly 

 elongated form of the skull iu the 

 child, and the small proportion which 

 the facial bears to the cranial part, 

 and also the interval left between the 

 parietal, occipital, and temporal bones 

 sometimes called lateral fontanelles. 

 The lines indicate the various dia- 

 meters. 



equally into two parts. Tlie pari- 

 etal region then increases rapidly 

 in volume, along with the in- 

 creased development of the cere- 

 bral hemispheres ; the frontal 



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

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

 bu-th 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 afterwartls. In the 



