94 A MANUAL OF VETERINARY PHYSIOLOGY 



for a larger amount of pressor substances may be recovered 

 from the blood leaving the gland than from ordinary blood. 

 Removal of the glands is followed by great weakening of the 

 heart, low blood-pressure, and fatal results ensue. 



Extracts of the posterior lobe of the pituitary body introduced 

 into the circulation greatly increase the tone but not the rhythm 

 of the heart muscle (see p. 59), and, with the exception of the 

 kidney, it causes constriction of the arterioles of other parts of 

 the body. This gland also secretes a depressor substance which 

 diminishes the force of the heart and permits the vessels to 

 dilate. Under experimental conditions the extract obtained 

 from the pituitary body contains much less of the depressor 

 than of the pressor substances, but it may well be that under 

 physiological conditions they are passed into the circulation in 

 the required proportion, and act, as in the case of the adrenals, 

 as a direct chemical stimulus. 



Muscular Aids. — Abdominal respiration assists the return of 

 blood from the splanchnic area. If an incision be made in the pos- 

 terior vena cava in front of the diaphragm, with every retreat of 

 that muscle pressure is exercised on the abdominal contents, and 

 the blood spurts out of the vessel. This pressure on the contents 

 of the abdominal cavity is one of the most important mechanical 

 aids to the circulation. The abdominal muscles are assisted by 

 the stout elastic fascia which plasters their external surface, the 

 anus is tightly closed, and the pressure within the abdomen is 

 raised at each inspiration. Failure to secure this pressure results 

 from a relaxed condition of the anus. This is frequently seen 

 in the last stage of some debilitating diseases in the horse. The 

 anus remains open, and allows the air to rush in and out of the 

 bowel at every respiratory effort. In the veins of the hind-limb 

 the whole course of the blood is directly against gravity until 

 the posterior vena cava is reached. In that vessel gravity is in 

 favour of the contents, for the vein falls from the spine to the 

 heart, and the respiratory help above described completes the 

 work as the heart is approached. The squeezing to which the 

 liver is exposed in the abdomen is a most valuable means of 

 assisting its circulation. 



Muscular movements of the body and limbs are of the utmost 

 importance as aids to the circulation. Muscles, by their con- 

 traction, squeeze the blood along the veins, regurgitation being 

 prevented by the valves. Hill has shown that the column of 

 blood in the vessels of the limbs may be broken up into segments, 

 and by muscular action the stream turned on or cut off to a 

 part, and the supply regulated according to its requirements. 

 The importance of muscular action as an aid to circulation is 

 seen every day, especially in the case of the horse. The swollen 



