30 



A MANUAL OF VETERINARY PHYSIOLOGY 



so constructed and arranged as to open only in that direction 

 towards which the blood has to be sent. 



Position of the Heart. — The heart occupies a position in the 

 middle line of the chest, and is suspended from the spine by 

 means of its arterial trunks. These are its only means of support. 

 Some help may be afforded by its connection with the root of the 

 lungs ; but in thinking of the heart as a pump it must be remem- 

 bered that all the movements it executes in its constant work 

 are carried out as the organ literally hangs from the spine. It 

 rests on nothing ; the apex is clear of the sternum ; the peri- 

 cardium keeps it in its place, but is no mechanical aid in keeping 

 it in position. In the dog the pericardium obtains attachment to 

 the diaphragm, but not so in the horse and ruminants. Further, 



Fig. ii. — Diagram of the Circulation of the Blood. 



The heart ; 2, anterior, 3, posterior aorta ; 4, anterior vena cava ; 5, pulmonary 

 artery ; 6, pulmonary veins ; 7. mesenteric arteries ;"8, intestinal capillaries ; 

 9, portal vein ; 10, theliver, the veins~irom which open into (12) the posterior 

 vena cava ; 11, the circulation through the hind extremities ; 13, the circula- 

 tion through the kidney. 



in the dog and cat the heart rests on the upper face of the sternum, 

 whereas in the horse it does not touch the sternum. Fig. 12 

 gives an accurate notion of the position of the heart. It will be 

 observed that the organ is tilted forward, the base lying in front 

 of the apex. 



The base of the heart is uppermost, and the organ in the horse 

 occupies a position corresponding to the third, fourth, fifth, and 

 sixth ribs. It is between the fifth and sixth ribs, just above 

 their sternal insertion, that the impulse of the heart can be felt. 

 On its right face is the right lung, and on its left part of the left 

 lung, the big triangular notch in which exposes the left ventricle 

 and enables it to make its impulse felt against the chest-wall. 

 The anterior face of the heart is formed by the right auricle and 

 ventricle, the posterior by the left auricle and ventricle. The 

 pulmonary "arteryTYuns along the left face, and the posterior 

 vena cava lies on the right face. 



