IGO 



PRINCU'LJ'JS OF ANIMAL JUOIJXIV 



neuron is connected with other neurons. Fif>;ure 1 19 represents a typical 



neuron. 



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



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



thus formed are supplied with blood vessels. 



The functions of the nervous system depend largely upon the manner 



in which the neurons are connected into neuronic systems and upon 



the character and positions of the terminations of the the axons and 



dendrites. These systems are of varying degrees of complexity. 



The Reflex Arc. — A simple reflex arc is composed of one sensory 



neuron and one motor (activator) neuron. 

 Such an arc is diagrammatically repre- 

 sented in Fig. 120. The example chosen 

 involves the spinal cord and a spinal nerve. 

 The dendrites of the sensory neuron l^ranch 

 out between the cells of the skin. The body 



Fig. 119. — Diagram of a typical 

 neuron, ox, axon; (/.dendrite; ms, 

 medullary sheath; vm, muscle; n, 

 node; ne, nerve endings; nu, nucleus 

 of cell of medullary sheath. 



Fig. 120. — Diagram of a simple reflex arc involv- 

 ing the spinal cord, aff, afferent fiber; dh, dorsal 

 horn of spinal cord; drg, dorsal root ganglion; eff, 

 efferent fiber; sk, skin; 7nf, muscle fiber; vh, ventral 

 horn of spinal cord. 



of the neuron lies in a ganglion, and its axon passes into the dorsal horn 

 of the gray matter of the spinal cord where its terminal branches come in 

 contact with the dendrites of the motor neuron. The body of the motor 

 neuron lies in the ventral horn of the gray matter and its axon emerges 

 from the ventral side of the cord, passes by the ganglion, and then runs 

 for some distance in the same bundle (nerve) with the dendrite of the 

 sensory neuron. The axon of the motor neuron terminates in a nuiscle in 

 the example chosen. In other similar reflex arcs it may end in a gland. 

 Stimulation of the sensory endings in the skin causes an impulse to 

 pass alojig the dendrite and axon of the sensory neuron to the motor cell 



