DISEASES OF THE HEART, BLOOD VESSELS, 

 AND LYMPHATICS. 



By W. H. Harbaugh, V. S. 



[Kevised in 1904 by Leonard Pearson, B. S., V. M. D.] 



The heart, blood vessels, and lymphatics may be described as the 

 circulatory apparatus. . 



The heart is located in the thoracic cavity (chest). It is conical in 

 form with the base or large part uppermost, while the apex, or point, 

 rests just above the sternum (breastbone). It is situated between 

 the right and left lungs, the apex inclining to the left, and owing to this 

 circumstance the heart beats are best felt on the left 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 com- 

 partments are called auricles and the lower ones are called ventricles. 

 The right auricle and ventricle are completely separated from the 

 left auricle and ventricle by a thick septum 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 opening, 

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

 force the blood into the arteries. In the interval between the con- 

 tractions 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 continued into 

 the veins and the arteries, forming their internal lining. The walls 

 of the ventricles are thicker than those of the auricles, 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 pericar- 

 dium, 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 ventricles as the pump 

 chambers. During the interval between contractions, 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; con- 

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