144 



PRINCIPLES OF ANIMAL BIOLOGY 



of fine projections which sometimes extend to great lengths. The cell is 

 compact in the embryo like most other cells, and the processes can be 

 seen to grow out from it, passing among other cells and dodging obstacles, 

 until they reach the organ to whose action they are to be related. These 

 projections are of two kinds, distinguished from one another not by struc- 

 ture but by their normal functioning. Those which normally conduct 

 impulses toward the body of the neuron are called dendrites; those which 

 convey impulses from the body of the neuron are axons. Figure 118 

 diagrammatically represents the parts of a typical neuron, and three 

 very different kinds of neurons are sho^vn in Fig. 119. 



These cells, which are strictly speaking the only constituents of the 

 nervous system, are bound together by connective tissue, and the masses 

 thus formed are supplied with blood vessels. 



Fig. 119. — Three kinds of nerve cells. A, from ventral horn of spinal cord of an ox; 

 B, from cortex of cerebrum of a cat; C, Purkinje cell from cerebellum of a cat; d, dendrite; 

 neu, axon; nu, nucleus; ntic, nucleolus. {B and C from Golgi preparations.) 



Functional Unit. — In the operations of a nervous system, the func- 

 tional unit is a group of neurons called a reflex arc. These neurons are 

 so related to one another that, following a stimulus or excitation, they 

 induce some sort of action. One end of the arc is in some tissue or 

 organ capable of receiving a stimulus, the middle of it is in the central 

 nervous system or an associated ganglion, and the other end of the arc 

 is in a tissue or organ capable of responding, such as a muscle or gland. 

 The arc consists of at least two neurons-, usually more. Leading from 

 the sense organ is a nerve fiber (neuron) which, on lacing stimulated, 

 conducts an impulse toward the central nervous system. This neuron 

 is called an afferent fiber, the name meaning literally "bearing toward" — 

 that is, toward the central system. It is also appropriately called a 

 receptor neuron; very commonly, also, it is called a sensory neuron, 

 though the result of the impulse it carries is not always sensation. The 

 opposite end of the reflex arc consists of a neuron whose tip is applied 



