THE HUMAN ORGANISM 



281 



225. The skeleton. Among vertebrates the skeleton, the 

 mechanical framework of the body, is an internal structure. 

 This is in contrast to the insects and other arthropods (crabs, 

 lobsters, spiders, etc.) and to the mollusks (clams, snails), 

 which have their skeletons on the outside of the body. The 

 general plan of the skeleton is that of the body as a whole. 

 There is a main axis, the 

 backbone, or vertebral col- 

 umn, extending back from 

 the brain box, or skull, and 

 there are two bony rings to 

 which the appendages are 

 attached— the pectoral, or 

 shoulder, girdle, and the 

 pelvic, or hip, girdle (see 

 Fig. 129). 



Bones of animals often 

 persist, buried in the earth, 

 long after the other tis- 

 sues have completely disap- 

 peared. As a result we have 

 remains of animals that 

 lived millions of years ago, 

 and that show many stages 

 between series that are liv- 

 ing today. Thus, there are 



skeleton remains, or fossils, of animals that resemble both birds 

 and reptiles. Again, in some animals bones develop during the 

 early stages of growth but never reach the condition of perform- 

 ing any function. This is true of leg bones in the whale, and the 

 legs of certain snakes, which never appear above the surface of 

 the body (see Fig. 132). 



The spinal-column units, or vertebrae (singular, vertebra), are 

 all built on very much the same plan (see Figs. 133 and 134). 

 Among fishes and some of the reptiles there may be an indefinite 

 number of vertebrae, but in the other classes of vertebrates there 



Fig. 134. The vertebrae and the nerves 



On each side of each vertebra, aa, passing 

 out between the stalks of the arches, are the 

 spinal nerves, bb. These nerves connect (i) 

 with the muscles and viscera at correspond- 

 ing levels, and (2) with the corresponding 

 sympathetic ganglia, cc 



