Diseases of the Heart and Organs of Circulation. 293 



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 arid one and a half inches 

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

 posed 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 di- 

 rected 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 later- 

 ally, 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 en- 

 veloped by lung tissue so that its exploration is about equally dif- 

 ficult 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 

 through 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 transverse musculo-membranous 

 partition having a large central orifice furnished with valves (au- 

 riculo- 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 reflex 

 when the ventricle contracts. The great artery which originates 

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

 tem of valves (semilunar) having their free borders turned 



