The Skeletal System. 



53 



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 corriposed of bone. It arises, however, embryonically in 

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

 CARTILAGE based on the primary condition in lower vertebrates 

 AND BONE. ^^ 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, 

 THE NOTO exhibit the notochord in 



CHORD. 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. 

 rmwpr>«!TTTnM 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 

 SUpraoccipital element, together forming an occipital ring en- 



FlG. 27. Transverse section 

 of shark vertebra (cartilage 

 stage), from young specimen of 

 Atlantic dogfish, Acanlhias. i, 

 intercalary cartilage, complet- 

 ing arch; n, notochord; v, body 

 of vertebra. 



