468 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cannot inspire great confidence because of the difficulty of distinguishing 

 between the results that follow the excitation of a nerve-path from or to the 

 centre and those following the excitation of the centre itself. According to 

 Laborde, who also used this method, the cardiac inhibitory centre is situated at 

 the level of the mass of cells known as the accessory nucleus of the hypoglossus 

 and the mixed nerves (vagus, spinal accessory, glosso-pharyngeal). 1 



The localization of the centre by the method of successive sections is per- 

 haps more trustworthy. Franck 2 has found that the separation of the bulb 

 from the spinal cord cuts off the reflexes called forth by nerves that enter the 

 spinal cord, while leaving undisturbed the reflex produced by stimulation of 

 the trigeminus nerve. 



On the whole, there seems to be no doubt that the cardiac inhibitory centre 

 is situated in the bulb. 



Tonus of Cardiac Inhibitory Centre. The cardiac inhibitory centre is prob- 

 ably always in action, for when the vagus nerves are cut, the heart-beat 

 becomes more frequent. The source of this continued or "tonic" activity 

 may lie in the continuous discharge of inhibitory impulses created by the 

 liberation of energy in the cell independent of direct external influences, or 

 the cells may be discharged by the continuous stream of afferent impulses 

 that must constantly play upon them from the multitude of afferent nerves. 

 This latter theory, the conception of a reflex tonus, is made probable by the 

 observations that section of the vagi does not increase the rate of beat after 

 the greater part of the afferent impulses have been cut off by division of the 

 spinal cord near its junction with the bulb, 3 and that the sudden decrease in 

 the number of afferent impulses caused by section of the splanchnic nerve 

 quickens the pulse-rate. 4 



Irradiation. The slowing of the rate of beat observed chiefly during the 

 expiratory portion of respiration disappears after the section of both vagus 

 nerves. The slowing may perhaps be due to the stimulation of the cardiac 

 inhibitory centre by irradiation from the respiratory centre. 5 



Origin of Cardiac Inhibitory Fibres. Since the researches of Waller 6 and 

 others, it has been generally believed that the cardiac inhibitory fibres enter 

 the vagus from the spinal accessory nerve, for the reason that cardiac inhibi- 

 tion was not secured in animals in which the fibres in the vagus derived from 

 the spinal accessory nerve were made to degenerate by tearing out the latter 

 before its junction with the vagus. These results have lately been called in 

 question by Grossmann. 7 The method employed by his predecessors, according 

 to him, probably involved the destruction of vagus roots as well as those of 

 the spinal accessory. Grossmann finds that the stimulation of the spinal 

 accessory nerve before its junction with the vagus does not inhibit the heart. 

 Nor does inhibition follow the stimulation of the bulbar roots supposed to be 

 contributed to the mixed nerve by the spinal accessory. 



1 Laborde, 1888, p. 415. 2 Franck, 1876, p. 255. 3 Bernstein, 1864, p. 654. 



4 Asp, 1867, p. 136. 5 Laulanie", 1893, p. 723. 



6 Waller, 1856, p. 420; Schiff, 1858; Heidenhain, 1865, p. 109; Giannzzi, 1872; Franck, 

 1876, p. 264. 7 Grossmann, 1895, p. 6. 



