258 ELECTRO-PHYSIOLOGY CHAP. 



canals for the most part opening against the wall of the body. 

 The second pair of capsules lies farther back, at the external wall 

 of the electrical organ. The system of Savian bladders consists 

 of saccules 23 mm. in diameter, which in life are perfectly 

 transparent. These occupy the four-sided space between the 

 anterior ends of the electrical organ as far as the upper lip, and 

 extend still farther backward. " Each sac consists of a homo- 

 geneous membrane of connective tissue, and is filled with a clear 

 gelatinous mass. The ingoing nerve forces its way through a 

 peculiar felted tissue, which lies like a cushion at the lower part 

 of the bladder ; it then divides into three branches, of which the 

 central is the strongest. Each of these forms a kind of expan- 

 sion, which supports the true sensory epithelium (hair -cells 

 resembling the auditory cells of the organ of Corti). In the 

 cervical region this structure is supplied by the trigeminal nerve, 

 in the region of the trunk by the vagus. 



After extirpation of brain and cord, the trigeminus (which 

 supplies the lateral ampullae and Savian vesicles) was dissected 

 out, the central end of the nerve, which is 23 cm. long, being 

 laid by its long and transverse sections across unpolarisable elec- 

 trodes ; there was then a distinct, if small, deflection on the 

 galvanometer, in the direction of a negative variation, each time 

 the skin was lightly compressed above the lateral packet of 

 Lorenzinian ampullse and Savian vesicles. It was subsequently 

 found that the last alone are responsible for the effect. 



This is, therefore, the second authenticated case in which 

 excitation of the peripheral end of a sensory nerve by adequate 

 stimuli produces negative variation of the demarcation current in 

 the divided trunk of the nerve. Obviously there is here a wide 

 and still unexplored field. 



The electromotive changes in the central endings of the 

 superior sensory nerves i.e. the sensory cortical regions in 

 consequence of adequate excitation of the peripheral sense-organ 

 (eye, ear), are of the greatest interest, although theoretically still 

 obscure : this is not, however, the place to discuss them. We 

 must now return to the negative variation of peripheral nerves 

 under artificial excitation. 



