112 



PHYSIOLOGY 



Structure of the Nervous System. — A nerve is made 

 of fibers that are prolongations of large irregular cells 

 (Fig. 68). Like the circulatory 

 system, the arrangement of the 

 nerves depends on the shape, size 

 and development of the animal. 

 In the starfish, for example (Fig. 

 69), it is star-shaped, for a ring 

 around the mouth sends a branch 

 to each ray. 



There are always one or more 

 central masses of cells from which 

 the fibers radiate. In worms and 

 lobsters these masses of cells are 

 arranged at intervals along a cord 

 that extends the entire length of 

 the body (Fig. 70). This is the 

 prototype of the spinal cord of 

 human beings. The spinal cord 

 is the fundamental part of the 

 vertebrate nervous system. It is 

 situated in a canal formed by the 

 vertebras where it is thoroughly 

 protected. From this cord nerves 

 are given off at regular intervals. 

 These subdivide into smaller and 

 smaller nerves which go to every 

 part of the body. At the anterior, 

 or head end, of the cord the cells 

 are massed into a large organ called the brain which 

 also gives off nerves in the same way. These go to 



Fig. 68. — Diagram of a mo- 

 tor nerve cell. C7., motor 

 nerve cell of spinal cord; 

 A., axis cylinder; C., col- 

 lateral branch ; 3fs. , me- 

 dullary sheath ; N., node ; 

 T., terminal branches ; 

 M., muscle. (From Fitz.) 



