Motor System — Muscles and Skeleton 101 



II. Skeleton 



The substance of living tissue is relatively soft. Minute aquatic 

 organisms may have no supporting or protective structures aside from 

 the delicate cell-membrane. Large animals would be impossible without 

 skeletons. Among invertebrates, with few exceptions, skeletal struc- 

 tures are external, consisting of hard substance secreted by the skin. 

 The shell of mollusks is calcareous. Arthropods have a cuticular 

 exoskeleton, consisting essentially of a secreted nitrogenous substance, 

 chitin. 



Such exoskeletons as those of invertebrates impose a limit on the 

 size of an animal. A lobster as large as an elephant would be mechani- 

 cally difficult, if not impossible — certainly impossible if it were neces- 

 sary for the elephantine arthropod to go through an occasional molting 

 stage leaving it temporarily in a "soft-shelled" condition. Many verte- 

 brates possess exoskeletal structures of one sort or another — the 

 calcareous scales of fishes; the dermal bony plates of some ancient 

 amphibians and of alligators, turtles and the armadillo; the thick, 

 horny scales of reptiles and of the mammalian pangolin. The super- 

 ficial dermal bones of the skull are, in a broad sense, exoskeletal. But 

 in the great majority of modern vertebrates there is no extensive de- 

 velopment of external skeletal structures to serve for mechanical sup- 

 port and protection. 



The present place held by vertebrates in the organic world and the 

 vast size and power attained by so many of them are due in no small 

 measure to the mechanical advantages accruing to them from the fact 

 that their essential and characteristic skeleton is internal. With in- 

 crease in the size of an animal, a supporting exoskeleton becomes in- 

 creasingly inefficient, but an endoskeleton may be reinforced and 

 amplified to make it adequate for the larger and heavier animal. An 

 ancient and basic mechanical, contrivance is the lever. The vertebrate 

 skeleton is, for the most part, a system of levers. To each of these power 

 is applied by muscles appropriately placed in relation to the fulcrum, 

 which is the joint at which the part moves. In the jointed legs of 

 arthropods, the muscles are inside the skeleton, an arrangement which 

 has mechanical limitations. The vertebrate leg, with its freely jointed 

 skeletal parts, and muscles working on all sides of a joint, achieves a 

 maximum of free mobility. 



In addition to the fact that the vertebrate skeleton is internal, it is 

 unique as to the nature of the materials of which it is constituted and as 

 to the manner of its embryonic development and its growth (see Chap. 

 7). Three quite different kinds of material enter into the composition 

 of the skeleton. The notochord consists of a soft, highly vacuolated 



