GENERAL CONSIDERATIONS. 371 



1001. The Cranial Nerves. As will be shown in Chap. XI, there are reasons for 

 regarding some at least of the nerves which arise from the brain as representing the motor 

 and sensory roots of ordinary myelonal nerves. 



1002, Functions of the Alba. So far as known, the nerves and other parts consisting 

 wholly of the white or fibrous nervous tissue are simply capable of transmitting impres- 

 sions which are made upon them ; their office is one of conduction only. 



1003. Functions of the Ginerea. Different portions of the gray or ganglionic nervous 

 tissue have been found to act in one or more of the following ways : 



(A) As Trophic Centers. The ganglia upon the dorsal roots of the myeloual nerve 

 trunks (Fig. 109) seem to preside in some way over the nutrition of those roots, and are 

 therefore said to have a trophic action. The same may be the case with the ganglia con- 

 stituting the central portion of the sympathic system. 



(B) As Centers of Automatic Action. The ganglia in the substance of the heart, and 

 perhaps in some other localities, appear to possess the power of bringing about the action 

 of the muscular fibers with which they are connected independently of other parts. 



(C) As Centers of Inhibition. Certain portions of the cinerea seem to be able, either 

 automatically or otherwise, to interfere with, check or inhibit the activity of other parts 

 of the cinerea. 



(D) As Agents of the Will. Portions of the cortex cercbri appear to be immediately 

 under the influence of volition, which is unable to directly actuate other parts of the 

 brain or of the body. 



(E) As a Medium of Conduction. Acting as the agent of the Will, the cortical cinerea 

 must be capable of transmitting impressions and impulses to and from the rest of the body. 

 Moreover, there are reasons for believing that the myelonal cinerea does, or at least may, 

 take a share in the transmission of impressions and impulses between the brain and the 

 body. 



(F) As a Center of Reflex Action. Many actions, especially those of the viscera and 

 vessels, are brought about in an indirect way. An impression made at the peripheral 

 end of a sensory nerve is transmitted through the nerve and through the dorsal or sensory 

 root to the cinerea of the myelon. Here it is apparently transformed into a motor im- 

 pulse which leaves by one or more ventral roots and causes an action of the appropriate 

 muscles or vessels or viscera. This kind of action, which may be very complex, is called 

 reflex, and the cinerea is said to operate as a center for the reception and interpretation of 

 the message and the determination of the response to be made. 



1004. Analogies of the Nervous and Vascular Systems. (A) Somewhat remotely f 

 the twofold division of the former into the (sympathic and the cerebro-spinal may be com- 

 pared with the division of the latter into the lymph and the blood vascular systems. 



(B) Confining the comparison to the better known and more extensive divisions, each 

 presents a central and a peripheral portion. In each case the central organs are relatively 

 active and the peripheral passive. 



(C) The vessels carry the blood, containing the elements of nourishment and the pro- 

 ducts of waste, together with heat. The nerves transmit impressions and impulses, by 

 means of which the different organs are connected and made capable of mutual and har- 

 monious action. Both vessels and nerves are thus channels of communication ; but they 

 differ as do rivers, canals, roads and railways from the telegraph and the telephone. The 

 relations established are of commerce on the one hand and intelligence on the other. 



(D) Again, as the vessels are of two kinds, the arteries which carry blood from the 

 heart and the veins which return it thereto, so there are two sets of nerves, motor and sen- 

 sory, which transmit impulses in one direction and impressions in the other. The direc- 



