172 COMPARATIVE PHYSIOLOGY. 



muscle ; or, long nucleated fibers transversely striped, covered 

 with an elastic sheath of extreme thinness, bound together 

 into small bundles by a delicate connective tissue, these again 

 into larger ones, till what is commonly known as a " muscle " 

 is fomied. This, in the higher vertebrates, ends in tough, ine- 

 lastic extremities suitable for attachment to the levers it may be 

 required to move (bones). Certain of the tissues will be found 

 briefly described in the sections preceding " Locomotion." 



Comparative. — ^The lowest animal forms possess the power 

 of movement, which, as we have seen in Amoeba, is a result 

 rather of a groping after food ; and takes place in a direction 

 it is impossible to predict, though no doubt regulated by laws 

 definite enough, ^f our knowledge were equal to the task of de- 

 fining them. 



Those ciliary movements among the infusorians, connected 

 with locomotion and the capture of food, are examples of a 

 protoplasmic rhythm of wonderful beauty and simplicity. 



Muscular tissue proper first appears in the Ccelenterata, but 

 not as a wholly independent tissue in all cases. In many 

 ccelenterates cells exist, the lower part of which alone forms a 

 delicate muscular fiber, while the superficial portion (myoblast), 

 composing the body of the cell, may be ciliated and is not con- 

 tractile in any special sense. The non-striped muscle-cells are 



most abundant among the in- 

 vertebrates, though found in 

 the viscera and a few other 

 parts of vertebrates. This 

 form is plainly the simpler 

 and more primitive. The 

 voluntary muscles are of the 

 ^'"•Sl^j:."au%.'^'"''"'''*'''*" striped variety in articulates 



and some other invertebrate 

 groups and in all vertebrates ; and there seems to be some re- 

 lation between thie size of the muscle-fiber and the functional 

 power of the tissue — the finer they are and the better supplied 

 with blood, two constant relations, the greater the contrac- 

 tility. 



Whether a single smooth muscle-cell, a striped fiber (cell), 

 or a collection of the latter (muscle) be observed the invariable 

 result of contraction is a change of shape which is perfectly 

 definite, the long diameter of the cell or muscle becoming 

 shorter, and the short diameter longer. 



