216 ANIMAL PHYSIOLOGY. 



cord in the cervical region produces paralysis of the blood- 

 vessels of the whole body; and other experiments show that 

 the great vaso-motor centre of all the vessels is situated in 

 the neighbourhood of the medulla oblongata; and that while 

 the vaso-motor nerves of the head and neck leave the cord 

 at the base of the neck to reascend in the sympathetic, those 

 to the rest of the body issue by the anterior roots of the 

 spinal nerves. 



The heart receives its nervous supply partly from the 

 sympathetic, and partly from the piieumogastric nerve. 

 Irritation of the sympathetic accelerates its action, as also 

 does irritation of the branch of communication between the 

 spinal cord and the inferior cervical ganglion of the sym- 

 pathetic. Thus the heart may be said to receive its 

 motor supply similarly to the arteries. But what is much 

 more remarkable is, that irritation of the pneumogastric 

 diminishes the frequency of the heart's action, and if 

 carried sufficiently far arrests it. This it does by pre- 

 venting in some way the normal impulse to contraction; 

 for the arrested heart has its walls relaxed, and contracts 

 on application of direct stimulus, and therefore the in- 

 hibitory action, as it is called, cannot be explained either 

 by spasm or exhaustion. 



There are other instances of similar inhibitory actions. 

 Thus, if one of the sensory nerves of a rabbit's ear be 

 divided, and its central end stimulated, the vessels of the 

 ear dilate; and in the case of the submaxillary salivary 

 gland of a rabbit, while irritation of the sympathetic con- 

 tracts the blood-vessels, irritation of another nerve dilates 

 them and increases the secretion. Such phenomena have 

 led to the use of the expression inhibitory nerves ; but the 

 use of that phrase must not lead it to be supposed that the 

 inhibitory action is in any case direct. The dilatation of the 

 vessels of the submaxillary gland is explained by the consider- 

 ation that nerves end in the secreting cells, and that increased 

 action of the cells may well attract more blood; and cases 

 which cannot be explained in a similar fashion may possibly 

 be all of them the result of action on nervous centres, and 

 not on terminal organs. It is not conceivable that nervous 

 impression could have two antagonistic effects on a muscle, 



