MODE OF INVESTIGATION. 497 



tion of which calls forth the activity of a gland : (y) inhibitory, 

 when they check or prevent some activity by the impulses which 

 they carry ; (<5) vasomotor nerves, which regulate the contraction 

 of the muscular coat of the bloodvessels ; (e) trophic, thermic, elec- 

 tric nerves are also to be named, the two former being of doubtful 

 existence, and the latter being only found in those animals which 

 are capable of emitting electric discharges, such as the electric 

 fishes. 



III. Intercentral nerves are those which act as bonds of union 

 between the several ganglion cells of the nervous centres, which 

 are connected in the most elaborate manner, one with the other. 

 As the terminals of these fibres are both probably receiving and 

 directing agents, the delicate strands of protoplasm communicating 

 between them probably convey impulses in different directions, 

 but of this we can have no definite idea, although such a supposi- 

 tion would aid us in forming a mental picture of the manner in 

 which the wonderfully complete intercentral communications are 

 accomplished. 



'MODE OF INVESTIGATION. 



In order to investigate the functions of the different nerves a 

 knowledge of their central connections and their peripheral distri- 

 bution is necessary. But anatomical knowledge,, however perfec^ 

 does not convey an adequate notion of their function, as may be 

 amply seen from deductions made by anatomists, many of which 

 have not borne experimental tests. 



The procedure adopted in testing the function of a nerve is the 

 following. The nerve is exposed and cut, and it is observed 

 whether there be any loss of sensation or muscular paralysis in 

 the parts to which it passes. The cut ends are then stimulated, 

 and the results are observed. The end of the part connected with 

 the centres is spoken of as the central or proximal end, and that 

 belonging to the part leading to the distribution of the nerve, is 

 called the peripheral or distal end. If the nerve be purely motor, 

 stimulation of the proximal end will yield no result, but when the 

 distal end be irritated, active movements follow. If, on the other 

 hand, it be a sensory nerve, stimulation of the distal end gives no 



42 



