MUSCULAR AND SKELETAL SYSTEMS. 



223 



indeed, is the only sign of its life. Any stimulus — me- 

 chanical, chemical, or electrical — may cause it to con- 

 tract, but the normal physiological stimulus is the nerve 

 influence, whatever that may be. When a muscle con- 

 tracts it shortens, thickens, and hardens. The power 

 with which it pulls in shortening is almost incredible. 



Involuntary Muscle. — We have already sufficient- 

 ly characterized this in contrasting it with the voluntary. 

 Good examples of these are found in the muscular coats 

 of the stomach, intestines, and bladder. Every one is 

 familiar with this type in the white muscular substance 

 of tripe. 



No comparative physiology of this system is neces- 

 sary, as the function, structure, and mode of action of 

 muscle is much the same in all animals, as far as the 

 tissue can be traced. Striation of muscular fiber is a 

 sign of great activity, and is therefore found only in 

 animals of considerable energy. It is conspicuous in 

 vertebrates, and among invertebrates in arthropods, 

 especially insects. As we go down the animal scale 

 muscular fiber is found as low as the ccelenterates — 

 medusae and polyps. In protozoa it is replaced by 

 general contractility of protoplasm. 



SECTION II. 

 Skeletal System. 



The skeleton in vertebrates is usually bone, because 

 this is the most rigid of the tissues. In the lower fishes, 

 however, and in the embryonic condition of all verte- 

 brates cartilage takes the place of bone. 



Bone is tissue hardened by deposit of lime salts, 



mainly phosphate. Several kinds of tissue take on this 



change. Thus, as to origin, we may have cartilage bone, 



tendon bone, and skin bone. Most of the true skeleton 



16 



