The Skeletal System. 



53 



CARTILAGE 

 AND BONE. 



THE NOTO- 

 CHORD. 



axial support. A corresponding vertebra of the bear will be found 



more or less intermediate between the two types (Fig. 25). 



The adult vertebra of the rabbit and of higher vertebrates in 



general is composed of bone. It arises, however, embryonically in 



cartilage (Fig. 26). The transition from cartilage to bone is 

 based on the primary condition in lower vertebrates 

 in which the entire skeleton, vertebrae included, 

 is formed in cartilage, and may remain in this 



condition throughout life. 



The axial line of the vertebrae in mammals passes through the 



central portions of the bodies. This position is marked, in the 



embryonic condition only, by the noto- 



chord (Fig. 23). Some of the lower 



aquatic vertebrates, such as lampreys, 

 exhibit the notochord in 

 both young and adult con- 

 ditions, and show little 



indication of the development of the 



elements of vertebrae. Others of slightly 



more advanced position, such as sharks 



(Fig. 27), show the notochord, extending 



more or less to the adult condition with 



the vertebral elements developed round 



about it. 



The head skeleton of a mammal, 



usually but inaptly called the skull, is a 



complex of individual bones and cartilage, 



the arrangement and functions of which may be determined with 



a little effort. The general disposition of the bone elements, 



demonstrable in the rabbit or any mammal is as indicated in Fig. 28. 



rniMPfiSTTTfiN Briefly, there is a linear series of basal segments, 



OF THE SKULL comprising from behind forward basioccipital, 



basisphenoid, presphenoid, and meseth- 



moid. The three first-named form the floor of the brain-case, 



while the mesethmoid forms the nasal septum. Associated with 



the basioccipital are paired, lateral exoccipital bones, and a 



Slipraoccipital element, together forming an occipital ring en- 



FiG. 27. Transverse section 

 of shark vertebra (cartilage 

 stage), from young specimen of 

 Atlantic dogfish, Acanthias. i, 

 intercalary cartilage, complet- 

 ing arch; n, notochord; v, body 

 of vertebra. 



