CIRCULATION OF THE BLOOD. 51 



CIRCULATION OF THE BLOOD. 



The Circulatory Apparatus by which the blood is distributed to all 

 portions of the body consists of a central organ, the heart, with which is 

 connected a system of closed vessels, known as arteries, capillaries and veins. 

 Within this system the blood is kept, by the action of the heart, in continual 

 movement, distributing nutritious matter to all portions of the body and 

 carrying waste matters from the tissues to the various eliminating organs. 



The heart is a hollow muscular organ, pyramidal in shape, measuring 

 about 5^ inches in length, about 3^ in breadth, weighing from 10-12 oz. 

 in the male and from 8-10 oz. in the female. Situated in the thoracic 

 cavity, between the lungs, its base is directed upward, backward and to the 

 right, its apex is directed downward and to the left. 



Pericardium. The heart is surrounded by a closed fibrous membrane 

 called the pericardium. The inner surface of this membrane is lined by a 

 serous membrane, which is also reflected over the surface of the heart ; 

 between the two surfaces of the serous membrane is found a small quantity 

 of fluid, the pericardial fluid, which lubricates the surfaces and prevents 

 friction during the movements of the heart. The interior of the heart is also 

 lined by a serous membrane called the endo-cardium. 



Cavities of the Heart. The general cavity of the heart is subdivided 

 by a longitudinal septum into a right and left half; each of these cavities is 

 in turn subdivided by a transverse constriction into two smaller cavities 

 which communicate with each other and are known as the auricles and 

 ventricles. The orifice between the auricle and ventricle being known as 

 the auriculo-ventricular orifice. The heart therefore consists of four 

 cavities, a right auricle and ventricle and a left auricle and ventricle. 



Into the right auricle, the two terminal trunks of the venous system, the 

 superior and inferior vence ca'vte, empty the venous blood which has been 

 collected from all parts of the system ; from the right ventricle arises the 

 pTilmonary artery which passing into the lungs, distributes the blood to the 

 walls of the air cells of the lungs ; into the left auricle empty {our pulmonary 

 veins which have collected the blood from the lung capillaries; from the 

 left ventricle springs the aorta, the general trunk of the arterial system 

 whose branches distribute the blood to the entire system. 



The Valves of the Heart. The valves of the heart are formed by a 

 reduplication of the endocardium strengthened by connective tissue. At the 

 auriculo-ventricular openings on the right and left sides of the heart respec- 

 tively are found the triciispid and mitral valves. The tricuspid valve 



