176 EQUINE PHYSIOLOGY. 



The number of pulsations of the heart per minute are from 

 36 to 40. In the new born horse they are 120, and at the end of 

 the first year from 48 to 56. 



Their relation to the respiratory movements are on an average 

 one to four (the number of respiratory acts being ten per minute, 

 usually) . 



The capacity of the ventricles is each about 20 ounces. 



The entire blood in the body would pass through the left ventri- 

 cle in 66 seconds in an animal of 1000 pounds weight, assuming 

 the amount of blood to be 55yV pounds {^-^ of the bodily weight), 

 and requiring 44.4 pulsations. 



Arterial Pressure is the force exerted by the blood on the ves- 

 sels, and is measured by a tube introduced into some of the large 

 vessels. In the carotid the blood pressure equals 32 mm. of 

 mercury; in the brachial 150 mm. 



The Arteries may be divided into three sets (Flint), the 

 largest ending at the carotid and iliacs, with elastic walls ; the 

 medium from these to yL of ^^ \x\c\i in diameter ; the smallest 

 from Y^ of an inch to the capillaries. The median are elastic 

 and contractile, the smallest only contractile. 



The velocity of the arterial current wd^iits in the different arte- 

 ries ; it is the greatest during systole, least during diastole. In 

 the carotid \X. is 300 mm. per second; in the maxillary 165 mm. 

 per second ; metatarsal 56 mm. 



The Capillaries form the communication between the arterial 

 and venous vessels, and the current of blood is much slower, only 

 To" ^o fV '^^'^' P^^ second, the blood pressure being only 35 mm. 

 In the capillaries the oxygen is absorbed by the tissues. 



The velocity of the venous current varies : in "the jugular vein 

 being 100 mai., and in the vena cava no mm. per second. The 

 blood pressure of the venous system is much less and the capacity 

 much more than the arterial. 



Vierordt showed that 31.5 seconds after injection of a salt solu- 

 tion into one jugular, it could be detected in the vein of the op- 

 posite side. 



The valves of veins prevent damming back of blood. 



The flow of blood is kept up in the veins — 

 I St. By the vis a tergo of the heart. 

 2d. By the vis a fronte or suction force of the thorax. 

 3d. By muscular contraction on the walls of the veins. 



