DISEASES OF CATTLE 115 



side of the chest behind the elbow. The heart may be considered as a 

 hollow muscle, 'Containing four compartments, two on each side. 

 The upper compartments are called auricles and the lower ones 

 are called ventricles. The right auricle and ventricle are com- 

 pletely separated from the left auricle and ventricle by a thick sep- 

 tum or wall, so that there is no communication between the right 

 and left sides of the heart. 



At the bottom of each auricle is the auriculo-ventricular open- 

 ing, each provided with a valve to close it when the heart contracts 

 to force the blood into the arteries. In the interval between the 

 contractions these valves hang down into the ventricles. 



The muscular tissue of the heart belongs to that class known 

 as involuntary, because its action is not controlled by the will. 



The cavities of the heart are lined by serous membrane, called 

 the endocardium. The endocardium may be considered as con- 

 tinued into the veins and the arteries, forming their internal lin- 

 ing. The walls of the ventricles are thicker than those of the auri- 

 cles, and the walls of the left ventricle are much thicker than those 

 of the right. 



The heart is enveloped by a fibrous sac (or bag), called the 

 pericardium, which assumes much of the general shape of the outer 

 surface of the heart. 



.The action of the heart is similar to that of a pump and its 

 function is to keep the blood in circulation. The auricles may be 

 considered as the reservoirs or receivers of the blood and the ven- 

 tricles as the pump chambers. During the interval between con- 

 tractions, the heart being in momentary repose, the blood pours 

 into the auricles from the veins; the auriculo-ventricular orifices 

 being widely open, the ventricles also receive blood; the auricles 

 contract and the ventricles are filled; contraction of the ventricles 

 follows; the auriculo-ventricular valves are forced up by the pres- 

 sure of the blood and close the auriculo-ventricular openings and 

 prevent the return of blood into the auricles; the contraction of 

 the ventricles forces the blood from the right ventricle into the 

 lungs through the pulmonary artery and its branches, and from 

 the left ventricle into the aorta and all parts of the body through 

 the arteries. After the contraction of the ventricles the heart is 

 again in momentary repose and being filled with blood, while the 

 valves in the aorta and pulmonary artery close to prevent the re- 

 turn of blood into the ventricles. 



The average weight of the heart of an ox is said to be from 

 3^/2 to 5 pounds, but, of course, the weight must be very variable 

 in different animals, owing to the many breeds and sizes of cattle. 



The vessels that convey the blood from the heart to all parts 

 of the body are called arteries; the vessels which return the blood 

 to the heart are called veins. Between the ultimate ramifications 

 of the arteries and the beginning of the veins there is an interme- 

 diate system of very minute vessels called capillaries, which con^ 

 nect the arterial with the venous system of the circulation. The 



