NERVES AND THE REACTIONS OF ORGANISMS 219 



Dorsal 

 root 



many of us are still capable), in the twitching of a horse's 

 skin when it is annoyed by flies, and in winking. And, finally, 

 we may recall the muscles of the esophagus, the stomach, the 

 intestines, and the 

 blood vessels. 



Some muscles 

 are called volun- 

 tary and some in- 

 voluntary, but all 

 muscles contract 

 in response to a 

 stimulus received 

 from a nerve cell. 



256. Nerves 

 and nerve cells. 

 The nerves that 

 are found run- 

 ning to all organs 

 of the body are 

 compounded of 

 many nerve fibers. 

 Many such fibers, 

 bound together 

 by connective tis- 



sue and associ- 

 ated with blood 

 vessels and lym- 

 phatics, constitute 



FIG. 83. Diagram of the spinal cord 



A, left half of cross section, showing impulses entering the 

 dorsal root and outgoing impulses passing out by the ventral 

 root. B, the neurons connected with the gray matter of the 

 cord give off branches passing up and down the cord and 

 transmitting nervous disturbances by way of the collaterals. 

 In the gray matter of the cord, branches of afferent neurons 

 carry impulses up and down and pass them on, by way of the 

 collaterals, to efferent neurons and to the brain 



a nerve. For our 



present purpose we are not so much concerned with the 

 nerves as we are with the nerve cells which compose them. 

 The nerve cell consists of (i) the cell body and (2) certain 

 processes, or outgrowths (fibers] ; together these make up a unit 

 of the nervous system. Such a unit is called a neuron, and may 

 be compared to a muscle cell as a unit of a muscle, or to a 



