THE ENCEPHALO-SPINAL MEMBRANES. 



493 



functions of the different neurons in their individual and collective capacity 

 the functions of the nerve system will become apparent. 



The Functions of the Nerve System. The functions of the nerve 

 system are twofold: (i) It unites and associates the organs and tissues of 

 the body in such a manner that they are enabled to cooperate for the accom- 

 plishment of a definite object. (2) It serves to arouse in the individual a 

 consciousness of the existence of an external 

 world, by virtue of the impressions which it 

 makes on his sense organs, and consequently 

 to enable him to adjust himself to his environ- 

 ment. 



By virtue of the anatomic and physiologic 

 association, a stimulus, if of sufficient inten- 

 sity, applied to one organ or tissue will call 

 forth activity in one or more organs near or 

 remote from the part stimulated. This coor- 

 dination of action is accomplished mainly by 

 the spinal cord and the medulla oblongata. 

 All actions which take place in response to a 

 peripheral stimulus and independently of voli- 

 tion are termed reflex actions. The reflex 

 activities connected with digestion, the circu- 

 lation of the blood, with respiration, excretion, 

 etc., are illustrations of the coordinating capa- 

 bilities of the nerve-centers located in these 

 portions of the central nerve system. 



Consciousness of the existence of the ex- 

 ternal world and of the relation existing be- 

 tween it and the individual is associated with 

 the physiologic activities of the encephalon, 

 and more particularly of the cerebral hemi- tenor root of spinal nerve. 5. 



Spheres. This portion Of the nerve system is Ligamentum dentatum. 6. Linea 



the chief, though perhaps not the sole, organ s P lendens " 

 of the mind, and its functions are for the most part mental. 



The function of a part at least of the peripheral nerve system is to afford 

 a means of communication between the central organs of the nerve system 

 and the remaining structures of the body. The nerve-trunks constituting 

 this part may be divided into two groups, as follows : 



1. The first group comprises nerves in connection with the special sense- 



organs, e.g., skin, eye, ear, nose, tongue, as well as nerves in connection 

 with the general or organic sense-organs, e.g., mucous membranes, 

 viscera, etc., which are connected primarily with nerve-cells in the spinal 

 cord and medulla oblongata, and secondarily with nerve cells in local- 

 ized areas of the cerebral cortex. 



2. The second group comprises nerves which terminate mainly in the 



muscle apparatus and which constitute the continuation of nerve paths 

 which have their origin in nerve-cells of localized areas of the cerebral 

 cortex. 

 The first group of nerves, the afferent, especially those connected with 



FlG> 228 . THE MEMBRANES OF 

 THE SPINAL CORD. i. Dura 



