512 MANUAL OF PHYSIOLOGY. 



and is adapted to the reception of the various external influences 

 which are brought to bear on it from without by its surroundings. 

 These receivers of extrinsic stimuli are necessarily much varied 

 so as to be capable of appreciating all the different kinds of stim- 

 ulation presented to them. They are either distributed over the 

 entire surface so as to meet with general mechanical and tHermic 

 changes, or they are further specialized for the reception of lumi- 

 nous, sonorous, odorous or gustatory impulses. In the latter cases 

 the special terminals are collected into one part, and usually form 

 complex organs, which will be described presently in the chapters 

 on the special senses. Another great set of terminals are placed 



FIG. 205. 



Tactile nerve endings, composed of small capsules, in which the black axis- 

 cylinder of the nerve (a) and (n) meets with many protoplasmic units. 



in the deeper textures, when they act as local distributing agents ; 

 such as the nerve plates on skeletal muscles, and the ganglionic 

 networks in the wall of the intestine. In many instances, how- 

 ever, the exact mode of connection between the nerve and the 

 protoplasm of the tissue elements, to which it bears impulses, has 

 not been satisfactorily made out. In the remaining class of nerve 

 terminals the cells are grouped together so as to form larger and 

 smaller colonies, and more definitely deserve the name of nerve 

 or ganglion cells. These are the central terminals, and are placed 

 either in the cerebro-spinal axis, or in swellings of the nerves 

 called sporadic ganglia. 



Of these nerve cells there are many varieties, all of which have 

 the following characteristics : The cells are of considerable size, 



