332 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



is divided into two cavities, having the rectum in the interspace. An artery is 

 continued from each extremity of the ventricle, which distributes the oxy- 

 genated blood over the viscera, the muscular system, and the mantle. The 

 heart of the Gasteropoda, likewise, consists of a single ventricle, which pro- 

 pels the blood to the viscera and the muscular system of the body, and 

 receives it from the branchiae by one, and sometimes by two, auricles. 



In the Cephalopoda, the most highly organised mollusks, the general plan 

 of the heart is the same as in the Gasteropoda. The venous blood is received 

 from all parts of the body by great venous sinuses, which also take up the 

 blood from the gills : these veins communicate with the heart, which consists 

 of a single cavity, whence arise the two main arteries of the body, called the 

 superior and inferior aorta. 



In Fishes, the heart consists of two cavities ; one, large, loose, and thin- 

 walled, which receives the venous blood the auricle; the second, thick and 

 fleshy the ventricle, whence an artery springs, the first portion of which, 

 dilated and surrounded by thick muscular fibres, constitutes what is called 

 the aortic bulb. 



In the Batrachian reptiles, there are two auricles ; one which receives the 

 blood from the veins of the body the systemic auricle ; the other, which 

 receives it from the lungs, the pulnionic auricle. Both auricles communicate 

 with a single ventricle, whence the blood is propelled throughout the body, 

 as well as to the lungs. In Serpents, the heart presents a similar structure ; 

 but in the Python, the ventricle is divided by an imperfect septum into two 

 chambers, one of vrhich communicates with the aorta, the other with the 

 pulmonary artery. In the Saurian reptiles, likewise, there are two auricles 

 and a ventricle, which latter is subdivided into two or more cavities, com- 

 municating with each other, and with certain arteries which spring from 

 them excepting in the American alligator (Crocodilus lucius), in which the 

 existence of a perfect septum creates two distinct ventricles. 



In Birds and Mammals, the heart exhibits its highest development, con- 

 sisting, as it does, of two auricles and two ventricles, separated by a complete 

 septum ; each auricle communicating \\ith its proper ventricle, and each 

 ventricle giving rise to an arterial trunk. 



The human heart, in the adult subject, occupies an oblique posi- 

 tion in the thorax. Its apex is directed downwards, forwards, 

 and to the left side, and in the quiescent state, corresponds to the 

 interval between the fifth and sixth ribs. Its base corresponds to 

 the interval between the third or fourth, and the eighth dorsal 

 vertebrae, from which it is separated by the parts contained in the 

 posterior mediastinum. The base of the heart corresponds in front 

 to the sternum, at about the level of the cartilage of the third rib. 



The weight of the human heart, in the adult, is about 1 1 ounces 

 for the male, and 9 ounces for the female (John Reid). 



The two great arteries, the aorta and pulmonary arteries, spring 

 from the base of the heart in front. Posteriorly, the base is formed 

 by the auricles. 



Both the anterior and the posterior surfaces of the heart are divid< 



