MODE OF INVESTIGATION. 499 



when they check or prevent some activity by the impulses which 

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

 of the muscular coat of the blood vessels; (e) trophic, thermic, 

 electric 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 communi- 

 cating between them probably convey impulses in different 

 directions; but of this we can have no definite idea, although 

 such a supposition 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 

 distribution is necessary. But anatomical knowledge, however 

 perfect, 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 result, and that of the proximal end produces signs of pain. 



