838 ELECTRO-PHYSIOLOGY 



P wave of the electro-cardiogram (the wave specially associated with 

 the passage of excitation over the auricle) is also slightly variable. 

 Roughly, the apex of the P wave may be taken as indicating the onset 

 of the auricular systole. The relation between the end of the auricular 

 systole, 4, and the electro-cardiogram, is still more variable, but in 

 general it falls not far from the summit of the R wave (a wave specially 

 associated with the passage of the excitation over the ventricle) . This 

 shows that the auricular systole still continues at a time when the ex- 

 citation process has already made considerable progress in the ventricle. 

 The mechanical contraction of the ventricle, as shown in Fig. 315, by 

 the position of the vibrations corresponding to the first sound, follows 

 very promptly the completion of the auricular systole. 



Central Nervous System. It was discovered by du Bois-Reymond 

 that the spinal cord, like a nerve, shows a current of rest between longi- 

 tudinal surface and cross-section, and that a current of action is caused 

 by excitation. Setschenow stated that when the medulla oblongata 

 of the frog was connected with a galvanometer, spontaneous variations 

 occurred which he supposed due to periodic functional changes in its 

 grey matter. Gotch and Horsley have made experiments on the spinal 

 cords of cats and monkeys. Leading off from an isolated portion of 

 the dorsal cord to the capillary electrometer, and stimulating the 

 ' motor ' region of the cortex cerebri, they obtained a persistent nega- 

 tive variation followed by a series of intermittent variations. This 

 agrees remarkably with the muscular contractions in an epileptiform 

 convulsion started by a similar excitation of the cortex, which consist 

 of a tonic spasm followed by clonic or phasic (interrupted) contractions. 

 By means of the galvanometer, the same observers have made in- 

 vestigations on the paths by which impulses set up at different points 

 travel along the cord. To these we shall have to refer again (p. 895). 



Electrical Phenomena of Glands. These have been studied with any 

 care chiefly in the submaxillary gland and in the skin, although the 

 liver, kidney, spleen, and other organs also 

 show currents when injured. In the sub- 

 maxillary gland the hilus is galvanometrically 

 positive to any point on the external surface 

 of the gland ; a current passes from hilus to 

 surface through the galvanometer, and from 

 surface to hilus through the gland (Fig. 316). 

 Fig. 316. Current of Sub- When the chorda tympani is stimulated with 

 maxillary Gland. rapidly - succeeding shocks of moderate 



strength, there is a positive variation i.e., the 



hilus becomes still more positive to the surface. This variation can 

 be abolished by a small dose of atropine. 



Skin Currents. So far as has been investigated, the integument of all 

 animals shows a permanent current passing in the skin from the external 

 surface inwards. This is feebler in skin which possesses no glands. In 

 skin containing glands the current is chiefly, but not altogether, secre- 

 tory. As such, it is affected by influences which affect secretion, a 

 positive variation being caused by excitation of secretory nerves e.g., 

 in the pad of the cat's foot by stimulation of the sciatic. The deflection 

 obtained when a finger of each hand is led off to the galvanometer, 

 which was at one time looked upon as a proof of the existence of currents 

 of rest in intact muscles, is due to a secretion current. 



Of more doubtful origin is the current of ciliated mucous membrane, 

 which has the same direction as that of the skin of the frog and the 

 mucous membrane of the stomach of the frog and the rabbit viz., from 

 ciliated to under surface through the tissue, or from ciliated surface to 

 cross-section, if that is the way in which it is led off. The current is 



