58 



OUTLINES OF EVOLUTIONARY BIOLOGY 



contraction of which serves to pump the blood through the blood- 

 vessels. It is more especially associated, however, with the 

 movements of the limbs, the bones of which form a system of 

 levers operated by the muscles which are attached to them. It 

 differs greatly in minute structure from unstriped muscle, though 

 consisting essentially of greatly elongated, nucleated fibres 

 endowed with remarkable powers of contraction. These fibres, 

 and the fibrillae into which they are subdivided, are characterized 

 by a transverse striation of alternate light and dark bands. 

 Their structure is very complex and in the fully developed 

 muscle it is difficult if not impossible to recognize the limits 



FIG. 22. Striped Muscle-Fibres (m.) from the Tail of a larval Axolotl, 

 showing their nuclei (n.), X 560. (From a photograph.) 



between the constituent cells. Fig. 22 represents a number of 

 striated muscle-fibres from the tail of a larval axolotl, in which 

 each fibre is seen to be provided with several distinct nuclei. 



The nervous system, as we have already pointed out, serves to 

 place the different parts of the body in communication with one 

 another and exercises a controlling and co-ordinating influence 

 over the whole, while through the mediation of the special organs 

 of sense it keeps the organism in close touch with its environ- 

 ment. The tissue of which it is composed (Fig. 23) consists 

 \)f nerve-cells and nerve-fibres, but the fibres are merely out- 

 growths of the cells. A cell and fibre together form a neuron 

 a single unit of the nervous system. The nerve-cells, or 

 rather their bodies, occur chiefly in the brain and spinal cord, 

 which constitute the central nervous system, but also in small 



