LECT. XVII. HEART. 311 



to the structure of the circulatory apparatus; but I shall 

 notice them in the briefest manner possible; and as we 

 ought to study the function in its most perfect and compli- 

 cated state, the few anatomical points which I shall here 

 notice will relate to the human body. 



Heart. The heart of man is a conical or pyramidal 

 cavity, formed by a kind of muscular sac divided into two 

 parts, each composed of two cavities, placed one above the 

 other ; one of these is called the ventricle, the other the 

 auricle. The two ventricles of the heart occupy the lower 

 portion ; and their cavities are much larger than those of the 

 auricles placed above them. The left auricle and ventricle 

 belong to the apparatus for the circulation of arterial blood ; 

 while the right auricle and ventricle belong to that for 

 venous blood. The ventricles have thicker walls than the 

 auricles, especially the left ventricle, from whence the blood 

 is propelled into the arteries, and into all parts of the body. 



The right ventricle and auricle communicate with each 

 other by an opening called the auricula-ventricular orifice ; 

 to the two sides of which is attached an annular membrane, 

 whose inner border is floating, and to which are fixed some 

 tendinous cords. The latter are attached to some muscular 

 bundles, or fleshy columns, which arise from the inferior 

 walls of the ventricle, and proceed towards the orifice : this 

 membrane is called the tricuspid valve. Another one, 

 analogous to this, and named mitral valve, exists at the 

 orifice by which the left auricle and ventricle communicate 

 with each other. It differs from the former by its greater 

 solidity and more considerable resistance, which are capable 

 of overcoming its tendinous filaments. The orifices of both 

 the left a^id right ventricles, through which the blood 

 escapes from the heart, are furnished with another kind of 

 valve of a very different construction to those already de- 

 scribed. They are called semilunar valves, and are formed 



