404 Veterinary Medicine. 



fifth rib. The part of the ventricle exposed extends about three 

 inches upwards from the apex, and is about two inches in its 

 transverse diameter. The great mass of the organ is covered by 

 lung substance. 



In the ox about the same extent of heart tissue is exposed. In 

 sheep a portion about an inch in height, and one and a half inches 

 in breadth, is left uncovered by lung. In the pig the heart is 

 exposed only in a triangular space of about an inch across. 



In the carnivora the heart lies more directly in the median line 

 of the chest. It appears as if tilted forward, so that its apex is 

 directed backward, and its base forward, while the body of the 

 organ lies directly over the breast bone. The lungs invest it on 

 both sides preventing any approximation to the walls of the chest 

 laterally, and it can best be auscultated by applying the ear over 

 the sternum. 



In birds the heart is situated in the centre of the chest and 

 enveloped by lung tissue so that its exploration is about equally 

 difficult at all points. 



The larger blood vessels at their origin from the heart are not 

 open to examination in the lower animals except to a limited ex- 

 tent in the dog. 



Internal arrangement and structure of the heart. In all 

 warm-blooded animals the heart is composed of two portions, the 

 internal cavities of which, are perfectly distinct from each other, 

 and contain blood in different conditions ; the right portion hold- 

 ing the impure, purple or venous blood which has just circulated 

 through the body, and the left portion being filled with the bright 

 crimson or arterial blood, which has been aerated by circulating 

 throiigh the lungs. Each of these portions is divided into two 

 distinct cavities, an upper (auricle) which receives the blood from 

 the veins, and a lower (ventricle) which receives the blood from 

 the auricle and transmits it into the arteries. The auricle is sep- 

 arated from the ventricle by a tranverse musculo-membranous 

 partition having a large central orifice furnished with valves 

 (auriculo- ventricular), the free borders of which are turned down- 

 ward, so that they allow the blood to flow freely downward from 

 the auricle, but completely close the orifice and prevent any reflux 

 when the ventricle contracts. The great artery which originates 

 from the base 'of each ventricle is likewise furnished with a sys- 



