DISEASES OF THE ORGANS OF CIRCULATION 65 



there are additional centres in the spinal cord. The peripheral 

 nerves which are connected with these centres possess in part the 

 function of contracting (vasoconstrictors, pressors) and in part 

 of dilating (vasodilators, depressors) the blood-vessels. The 

 peripheral vessels also appear to possess independent vasomotor 

 ganglion cells. 



The vasomotor nervous apparatus can be stimulated or de- 

 pressed by agents acting centrically or peripherally. The direct 

 centric stimulants (vasoconstrictors) are digitalis, strychnine, 

 nicotine and carbon dioxide; insufficient ojcygen in the blood has 

 the same effect. The depressants or vasodilators are alcohol, 

 ether, chloroform and amyl nitrite. Contraction and dilation of 

 the vessels are also produced directly or indirectly (reflexly) by 

 different agents acting peripherally (mechanical, chemical, thermic, 

 and electrical stimuli applied to the skin or mucous membranes). 

 AdrenaUn is believed by some to cause vasoconstriction by centric 

 stimulation, others attribute its action to a peripheral stimulant 

 effect upon the vessels. 



Contraction of the vessels increases while dilatation decreases the 

 blood-pressure. The normal blood-pressure in the carotid artery 

 is equivalent to 150 millimetres of mercury; in the aorta to 200 to 

 250 millimetres. The greatest increase in blood-pressure is pro- 

 duced by increase of the heart action and contraction of the blood- 

 vessels simultaneously (digitalis, strophanthus). An increase in 

 blood-pressure also occurs when areas supplied by the larger vessels 

 are shut off from the circulation and when there is any interference 

 with the circulation of the blood in the larger arteries (insufficiency 

 of the aortic valves, thrombosis of the pelvic or femoral arteries). 

 In the latter conditions the continued high blood-pressure results 

 in a compensatory hypertrophy of the heart. 



The rate of flow of the blood in the carotid artery of the horse 

 is, on the average, 300 millimetres. It is dependent upon the 

 activity of the heart, the contraction of the blood-vessels and the 

 depth of respiration (aspiratory effect of deep inspiration). 



The causes of the coagulation of the blood have not been satis- 

 factorily explained. The fibrin which is in solution in the blood 

 5 



