584 SIGNIFICANCE OF THE NERVOUS SYSTEM 



cell. The former gives rise to motion and the latter to secretion. 

 It must be remembered, however, that the muscle cell presents itself 

 in three forms, giving rise respectively to the striated, non-striated 

 and cardiac tissue, and furthermore, that especially the second type 

 of muscle cell is a constituent of a most perplexing array of structures. 

 Thus, we find it in the iris, ciliary body, stomach, intestines, blood- 

 vessels, ureter, bladder, sexual organs, skin, etc. In all these cases 

 it responds to stimuli by contracting, but the effect produced thereby 

 differs greatly in accordance with the general arrangement of the tissue 

 in which it is embedded. Clearly, the movement shown by the iris, 

 is different in character from that of the contracting stomach or blad- 

 der. The same is true of the gland cell. While representing only 

 one type of effector, this cell appears in various forms as a unit of the 

 multitude of the glandular structures present in our body. Its stimu- 

 lation, therefore, must give rise to secretions of very different appear- 

 ance and composition. Thus, while it is customary to illustrate reflex 

 action with the help of motion, and especially with that type of it which 

 is caused by striated muscle, it should not be forgotten that the body 

 is also in a position to give a multitude of secretory responses. 



The conditions met with on the afferent side of the reflex circuit, 

 betray a much greater diversity of structure and function. The 

 layman commonly states that there are five sense organs present in 

 our body, namely the eyes, ears, nose, tongue and skin. We shall 

 find, however, that these five so-called external senses are augmented 

 by about twenty others which are chiefly concerned with the impres- 

 sions brought to bear upon internal parts. It appears, therefore, that 

 the two effector units, the muscle and the gland cell, are opposed by 

 more than twenty receptors, every one of which presents very special 

 structural characteristics. Motion or secretion are thus given in 

 answer to sensory impressions received from a relatively great number 

 of diversified receptors. 1 



The Reflex Animal. In studying reflex action, it is customary to 

 make use of a frog, the brain of which has been removed or destroyed 

 by the process of pithing. Obviously, this procedure destroys the 

 "psychic" life of this animal and renders its actions absolutely non- 

 volitional. An animal Of this kind, therefore, is incapable of ex- 

 periencing pain or of receiving any other sensation in consciousness. 

 In the absence of the cerebrum, an afferent impulse must necessarily 

 remain a simple reflex sensation. The removal of the cerebral hemi- 

 spheres, therefore, serves the purpose of converting the frog into a 

 simple reflex animal. 



The reflexes commonly studied subsequent to this procedure, are 

 those occurring in the domain of the spinal cord, i.e., the so-called 

 spinal reflexes. The frog is suspended from a hook passed through 

 its lower maxilla. The sole of the foot is then stimulated either by 



1 A further discussion regarding the structure of receptors will be found upon 

 page 729. 



