334 



PHYSIOLOGY 



CHAP. 



it cannot depend on the centrifugal nerves of the cardiac plexus. 

 After dividing the splanchnic nerve the depressor effect is much 

 less. The vascular dilatation must, therefore, take place largely 

 in the vessels controlled by this nerve, and only in a minor degree 

 in other vessels. 



These results were confirmed and extended by various observers 

 on the cat, horse, dog, and pig. In poikilothermic animals no 

 separate depressor nerves could be discovered. 



FIG. 154. Exposed nerves of rabbit's neck. (Doyon and Morat.) nv, Vagus nerve ; ml, depressor 

 nerve, arising above in two branches, one given off from the vagus trunk, the other from the 

 superior laryngeal ; nls, superior laryngeal nerve ; nil, inferior or recurrent laryngeal nerve ; 

 hes, external branch of spinal nerve ; ni, hypoglossal ; gcs, superior cervical ganglion ; sc, cervi- 

 cal sympathetic ; gci, inferior cervical ganglion ; etc, carotid artery ; acts, axillary ganglion ; 

 md, digastric muscle ; msi, stylo-hyoid muscle. 



Attempts were made to determine the exact peripheral distri- 

 bution of the depressor nerve. Wooldridge and Kazem-Beck 

 believed that its fibres entered the walls of the ventricle ; more 

 recently Kb'ster and Tschermak (1902) discovered that its nerve- 

 endings lay in the wall of the aorta. 



It is highly probable that the depressor (which, as we have 

 said, is not in tonic excitation) is only excited when the pressure 

 in the aorta becomes excessive, and obstructs the systolic evacua- 

 tion of the heart. The vessels then dilate, pressure falls, and the 



