176 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



Bernstein found that the afferent nerves in Goltz's experiment were branches 

 of the abdominal sympathetic, and discovered that the stimulation of the cen- 

 tral end of the abdominal sympathetic in the rabbit was followed also by reflex 

 inhibition of the heart. 



The stimulation of the central end of the splanchnic produces a reflex rise 

 of blood-pressure and, perhaps secondarily, a slowing of the heart. In some 

 cases acceleration has been observed. According to Roy and Adami splanch- 

 nic stimulation sometimes produces a combination of augmentor and vagus 

 effects, the augmentation appearing during stimulation and giving place 

 abruptly to well-marked inhibitory slowing at the close of stimulation. 



The results of stimulating various abdominal viscera have been studied by 

 Mayer and Pribram. One of the most interesting of the reflexes observed by 

 them was the inhibition of the heart called forth by dilating the stomach. 



The stimulation of the cervical sympathetic does not give any very constant 

 results on the action of the heart. 



B. The Centres op the Heart-nerves. 



Inhibitory Centre. — It has been already mentioned that the brothers 

 Weber localized the cardiac inhibitory centre in the medulla oblongata. The 

 efforts to fix the exact location of the centre by stimulation of various parts, 

 either mechanically, by thrusting fine needles into the medulla, or electrically, 

 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). 



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

 haps more trustworthy. Franck 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 Card/in- 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. 1 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 docs not increase the rate of beat after 

 the greater part of the afferent impulses have been cul oil* by division of the 

 1 Hunt : American Journal oj Physiology, 1899, ii. p. 397. 



