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 composed 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. 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, 
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 Tee Hine ASAD Ca 
b C of shark vertebra (cartilage 
about it. stage), from young specimen of 
Atlantic dogfish, Acanthias. i, 
The head skeleton Ona mammal, intercalary cartilage, complet- 
usually but inaptly called the skull, is a ig arch jt notochord wy body 
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. 
COMPOSITION Briefly, there isa 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- 
