460 VETERINARY PHYSIOLOGY 



abdominal cavity, but also througbout the body generally. 

 This is the most generalised vaso-dilator reflex known (see 

 p. 418). 



(6) Stimulation from the Cerebrum. — Not only does peripheral 

 stimulation thus act reflexly, but various states of the brain, 

 accompanied by emotions, may stimulate part of the vaso- 

 dilator mechanism, as in the act of blushing. 



The vaso-constrictors and vaso-dilators have a reciprocal 

 action, and this is of the greatest importance in physi- 

 ology and pathology. It explains the increased vascularity 

 of a part when active growth is going on. The changes 

 in the part, or the products of these, stimulate the afferent 

 nerve. This reflexly stimulates the vaso-dilator mechanism 

 of the part, and thus causes a free flow of blood into the 

 capillaries, and, at the same time, maintains or actually 

 raises the arterial pressure by causing a general constriction 

 of the arterioles, and thus forces more blood to the situation 

 in which it is required. It also explains the vascular changes 

 in inflammation (fig. 192). 



This reciprocal action may be disturbed just as the 

 reciprocal action of motor and inhibitory nerves in reflex 

 action may be disturbed. The administration of strychnine, 

 which in reflex action converts inhibitory into motor responses, 

 also converts vaso-dilator into vaso-constrictor responses, while 

 chloroform tends to convert vaso-constrictor into vaso-dilator 

 actions. 



(3) Position of the Centimes.— While the dominant vaso- 

 constrictor centre is in the medulla, the vaso-dilator centres 

 seem to be distributed in the medulla and spinal cord. 

 The vagus is the great outgoing vaso-dilator nerve from the 

 centres in the medulla, and the nervi erigentes, or pelvic 

 nerves, from the sacral part of the cord. 



11. Pressure in the Capillaries. 



This may be determined (a) by finding the pressure 

 required to blanch the skin or to occlude the capillaries of 

 some transparent membrane, or (6) by inserting a hypodermic 



