632 



SPECIAL PHYSIOLOGY. 



THE HEART. 



The Heart and Bloodvessels. 



The heart, inclosed in its sac, or pericardium, is placed obliquely 

 in the thorax, between the lungs (Fig. 13), occupying a space about 

 4 inches in width. It is of a conical shape. Its base, connected with 

 the large bloodvessels, is directed upwards, backwards, and to the 

 right, corresponding with the middle of the dorsal region ; its apex 

 turned downwards, forwards, and to the left, points to the left of the 

 sternum, opposite the interspace between the fifth and sixth ribs, two 

 inches below and one to the sternal side of, the left nipple. 



Its anterior surface, turned slightly upwards, is convex ; its pos- 

 terior surface, directed downwards, and supported by the diaphragm, 

 is flattened. This organ is about the size of the closed fist. In the 

 adult male it weighs from 10 to 12 oz., but from 8 to 10 oz. only in 

 the female. Its proportion to the body, in the former sex, is as 1 to 

 169 ; in the latter, as 1 to 149. It measures about 5 inches in length, 

 3J in width, and 2J in thickness. It increases in weight, and enlarges 

 in all its dimensions, as life advances. 



The heart is a hollow muscle, its cavity being completely divided 

 internally, by a longitudinal septum, into a right and a left lateral 

 chamber. Each chamber consists of two cavities, one called an au- 

 ricle, the other a ventricle, marked off from each other by a trans- 

 verse constriction, which forms on the surface the auriculo- ventricular 

 groove. The auricle and ventricle of the same side open into each 

 other, but those of the opposite sides do not communicate. The two 

 auricles are placed at the base of the heart ; their walls are thin; they 

 are separated from each other by the median septum, and receive 

 blood from large veins. The two ventricles lie below the auricles, have 

 walls of considerable thickness, and form the most solid part of the 

 organ ; each is connected with a large artery. Two longitudinal fur- 

 rows, one anterior, the other posterior and less defined, correspond 

 with the position of the median partition which separates the two ven- 

 tricles within. The right ventricle occupies more of the anterior, 

 and the left ventricle more of the posterior, surface of the heart ; the 

 left ventricle reaches lower than the right, and so forms alone the 

 apex of the heart, the longitudinal furrows and septum terminating a 

 little to the right of the apex. Each of the four cardiac cavities re- 

 quires further description. 



The right auricle (Fig. 105, 3) consists of a larger part, named the 

 sinus, and 1 a smaller part leading from it in front, named the appendix 

 auriculce or proper auricle, so called on account of its resemblance to a 

 dog's ear. The margins of the appendix are notched, and its walls, 

 instead of being thin and smooth, like those of the sinus, are thick, 

 and marked internally by prominent fleshy bands, the musculi pec- 

 tinati. Into this auricle the systemic veins open, viz., the superior 

 vena cava, 1, at the upper and forepart of the sinus; the inferior vena 

 cava, 2, at its lowest part ; and, lastly, the large coronary vein at the 



