240 



1'IIYSIOLOGY FOR DENTAL STUDKNTS. 



receptor cell, as it is called, is prolonged inside the animal as a 

 fiber, the nerve fiber, which passes to effector cells specialized 

 either as muscle fibers or gland cells. When a stimulus acts on 

 the receptor cell it therefore sets up a nerve impulse which causes 

 effector cells to become active, so that the animal either moYes 

 away or prepares to defend itself by secreting some poisonous 

 substance or making some defensive movement. There are, how- 

 ever, very few, even of the lowliest organisms, which have so 

 simple a nervous system as this, for the nerve fibers from differ- 

 ent receptors usually join together to form a nerve plexus and 

 they do not run directly to the effector cell, but to another cell. 



Fig. 39. Schema of simple reflex arc ; r, receptor in an epithelial mem- 

 brane ; a, afferent fiber ; s, synapsis ; c, nerve cell of center ; e, efferent fiber ; 

 in, effector organ. 



the central nerve cell, which is specialized as a junctional or dis- 

 tributing center, and which then transmits the impulse by a fiber 

 of its own to the proper effector organs. 



Thus we have the essential elements of the so-called rcfl< .r <irc 

 (Fig. 39), that is, a receptor connected with a nerve fiber called 

 <ijj( rent running to a central nerve cell which is again connected 

 with a nerve fiber called efferent, which passes to some effector 

 organ. In certain of the lower organisms these nerves and ner\e 

 cells are continuous throughout, but in the higher animals tin- 

 fibers originating from each cell do not actually join with those 



