THE VERTEBRATE BODY 



139 



that its study, known as osteology, forms an important sub- 

 division of comparative anatomy. 



In the lower Fishes the endoskeleton is composed of a firm elastic 

 tissue, cartilage, or gristle, but from the higher Fishes to Man 

 most of the cartilage becomes ossified: that is, impregnated 

 with lime salts and transformed into bone. The human skeleton 

 is formed of about 200 separate bones, but the number varies at 

 different periods of life, because some bones which at first are 

 distinct later become fused. (Figs. 103-105.) 



While it is true that the bones constitute the main supporting 

 framework of the body, they are entirely inadequate to knit to- 

 gether the organism into a working unit. We find therefore various 

 kinds of connective tissue interwoven between the integral parts 

 of the body. These tissues form sheaths about most of the organs 

 and also supply the connecting links between muscle and muscle, 

 muscle and bone — tendons ; and bone and bone — ligaments. 

 Supporting tissues, of which bone, cartilage, and connective tissue 

 form the chief groups, are characterized by the development of 

 large amounts of resistant non-living material in or between the 



Neural spine 



Transverse 

 process 



Vertebral 

 canal 



Neural 

 arch 



Anterior 

 articulating 

 surfaces 



Centrum 



i-e-h. 



Fig. 101. — A typical human vertebra (tenth thoracic). 



component cells themselves ; the character of the tissue being de- 

 termined chiefly by the nature of this matrix. (Fig. 32.) 



The primitive axis of the skeleton consists of a cylindrical cord 

 or rod of cells (notochord), which lies in the mid-dorsal line of 

 the body wall just below the brain and spinal cord and above 

 the coelom. In most Vertebrates, however, the notochord in its 



