682 



THE SKELETAL SYSTEM 



TRABECULA CRANII 



CENTRAL STEM 



SPHENOLATERAL 

 (ORBITAL) CARTILAGE 



HYPOPHYSIAL STALK 

 HYPOPHYSIAL CARTILAGE-.. 

 POLAR CARTILAGE 

 INTERNAL CAROTID ARTERY 

 NOTOCHORD 



PARACHORDAL CARTILAGE 

 AUDITORY CAPSULE 



Fig. 320. Diagrams of basic cartilaginous underpinning or foundation of the vertebrate 

 neurocranium. (Somewhat modified from De Beer, '37, after De Beer and Woodger.) 

 (A) Pisces. (B) Placenta! mammals. It is to be observed that the trabecula cranii in 

 the fish is represented by the central stem or trabecular plate in the mammal. 



most amphibia and in the reptiles, birds, and mammals, the notochord tends 

 to be entirely displaced by the vertebrae, and its residual remains are restricted 

 within or between the vertebrae. In mammals, the residual remainder of 

 the notochord constitutes the nucleus pulposus (pulpy nucleus) near the 

 center of the fibrocartilage of the intervertebral disc. In the human, according 

 to Terry, '42, p. 288, the pulpy nucleus forms a "pivot round which the bodies 

 of the vertebrae can twist or incline." 



b) Vertebrae. Vertebrae, the distinct segments of which the spinal column 

 consists, arise from sclerotomic mesenchyme, derived from the ventro-mesial 

 aspects of the various somites (fig. 252A-D). Potentially, this sclerotomic 

 mesenchyme in each primitive segment becomes segregated into eight masses, 

 four on either side of the notochord. These eight masses or blocks of mesen- 

 chyme form the arcualia. The arcualia become arranged in relation to the 

 notochord and the developing intermuscular septa as indicated in figure 321 A. 

 These masses are designated as basidorsals and basiventrals, interdorsals and 

 interventrals. Thus there are two basidorsals, two basiventrals, two interdor- 

 sals, and two interventrals. 



During the formation of the vertebra in mammals, the sclerotomic masses 

 within a primitive body segment become associated about the notochordal 

 axis as indicated in figure 321J-L. It is to be observed that the arteries from 

 the dorsal aorta lie in an intersegmental position. This position represents 

 the area of the myoseptal membrane, shown in figure 321 A. As the scle- 

 rotomic masses increase in substance, each mass on each side of the noto- 

 chord becomes divisible into an anterior area, in which the mesenchymal cells 

 are less dense, and a posterior area, where the cells are closely aggregated 



