92 A MANUAL OF VETERINARY PHYSIOLOGY 



this centre to the vessels, some of a constrictor, others of a 

 dilator nature, resulting either from the afferent impulses received 

 or as the result of changes in the composition of the blood 

 circulating through the medulla itself. 



Absence of Vasomotor Nerves. — Some vessels have no vaso- 

 motor nerves. They have not been satisfactorily demonstrated 

 in the vessels of the heart, nor in those of the lungs or brain. 

 They are not found in veins (excepting the portal, which is 

 really an artery), and are only provisionally accepted as 

 occurring in muscles. If muscles possess vasomotor nerves, 

 they must be essentially dilators, and it is easy to understand 

 that these fibres may be brought into operation when the 

 muscles contract, and thus provide a flow in proportion to the 

 activity of the part. It has, however, been suggested that the 

 dilatation in the bloodvessels of muscles during activity may 

 be due to the chemical action of acid metabolic products on 

 the vessels themselves. 



Surgical Shock. — The condition known as shock, which follows 

 certain operations, is intimately connected with the vasomotor 

 apparatus. This has been suggested in speaking of the influence 

 of carbon dioxide in the blood in maintaining blood-pressure. 

 In surgical shock there is a marked depressor effect on the cir- 

 culation, and a falling blood-pressure is one of the earliest indica- 

 tions of its onset. The maintenance of blood-pressure is the 

 cardinal principle in combating shock, yet the physiologist does 

 not regard shock as beginning in the vasomotor centre, as has 

 been suggested, in consequence of the violence of sensory stimu- 

 lation, but rather in the paralysis of the nervous connections 

 with that centre. The cutting off of the afferent impulses which 

 stimulate the vasomotor centre is the initial move towards the 

 production of shock. 



Aids to the Circulation. — The distribution of the blood supply 

 in accordance with the requirements of the various organs is the 

 special duty of the system of nerves we have been studying. 

 The requirements vary from time to time, even from moment to 

 moment. Active digestion diverts the blood to the splanchnic 

 area ; active work causes the stream to pass into muscles, and 

 later into the skin, in order to get rid of the excess of heat. 

 Some glands, such as the kidneys, are secreting constantly ; 

 others, like the pancreas, intermittently. A failing heart requires 

 more blood, one struggling against an ever-increasing load re- 

 quires less, and so on. Throughout the whole life of the animal 

 this delicate balance has to be maintained, and it is effected by 

 means of the nervous system through the medium of the heart 

 and bloodvessels. 



It is impossible in connection with this question of an efficient 



