THE HEART. 217 



the heart, from the point to the root of the vessels. But 

 this occupies only a part of the bladder. The rest is brought 

 down towards the sternum, and then up again to join the 

 point whence it started. This serous sac is externally covered 

 by a white fibrous membrane, which is continuous with 

 the cellular investment of the vessels, and is reflected over 

 the serous layer down to the sternum ; the whole being 

 within the pleurae, and called the pericardium. 



The heart. — This important organ is a hollow involun- 

 tary muscle, or pulsating sac, divided into four principal 

 cavities. Invested by its sac, it is situated obliquely; its 

 base being on a line with the dorsal vertebrae, and its point 

 directed to the left of the sternum between the eighth and 

 ninth ribs {Plate VIII. D). It presents four cavities, two 

 inferior or ventricles, and two superior or auricles. Each 

 ventricle opens by two spacious orifices : one of which com- 

 municates with the auricle or upper cavity ; the other forms 

 the mouth of a large artery. There are an auricle and a 

 ventricle upon each side of the heart ; or one lower and 

 one upper cavity upon the left, and the same upon the 

 right ; or, more properly, one anteriorly and one posteriorly 

 situated. The cavities are separated by a fleshy division, 

 like to the party wall between two houses, called a sep- 

 tum. The light ventricle has within it fleshy pillars, called 

 carnea columiKS, from which proceed tendinous cords, or 

 the cortina tendinea, attached to the three valves between 

 the right auricle and ventricle, which answer the purpose 

 of keeping each cavity distinct ; the left or posterior ven- 

 tricle, although altogether longer than the right, reaching to 

 the very apex of the organ, is, notwithstanding, less capa- 

 cious, but altogether of much greater substance. The 

 auricles are separated from each other by a septum, though 

 externally they appear altogether one cavity, with indented 

 edges. The right auricle is irregularly rounded, and has 

 two large openings for blood, which are the entrances of 

 the vena cavas. The left auricle is considerably more 

 muscular, though its size is less : it receives the pulmonary 

 veins, usually from three to five in number, and it has also a 

 common opening with the ventricle. The substance of the 

 auricles is both membranous and muscular, and they are 

 also furnished with a few tendinous and fleshy cords to 



