THE HEART. 221 



THE HEART. 



In all animals the existence of life is connected with, or rather 

 dependent upon, the constant supply of fresh arterial blood. There 

 is not a secretion that can be performed, or a function discharged, or 

 a single motion accomplished, without the presence of this vital fluid. 

 The heart is the grand engine by which it is circulated through the 

 frame. Tt is a large muscle, or combination of muscles, totally 

 independent of the will, and working without cessation and without 

 fatigue, from the first day of existence un{il its close. It is the 

 forcing pump by which the vital current, having completed its course, 

 is made to flow again and again to every part of the system. It 

 consists of four cavities, surrounded by muscular Avails, which, stimu- 

 lated by the organic nerves, can contract upon, and drive out, and 

 propel forward, the fluid which they contain, and then, left to them- 

 selves, can instantly re-assume their open dilated state by their inhe- 

 rent power of elasticity. 



A portion of the blood has completed the circulation, and enters 

 the upper cavity of the heart — the right auricle — where it accumu- 

 lates as in a reservior, until there is enough to fill the second and 

 lower cavity on the same side — the right ventricle — when the 

 auricle suddenly contracts and drives the blood forward into the 

 ventricle. But this blood is in a venous state, having just come from 

 the veins, and will not support life ; then it must change its character 

 before it is thrown back again into the circulation. It must be con- 

 veyed into the lungs, there to be exposed to the influence of the 

 atmospheric air, and purified, and arterialized. For this purpose, the 

 ventricle, stimulated by nervous energy, contracts, and as it con- 

 tracts, it drives a little of the blood back, but it forces more under a 

 dense fringed membrane which hangs around the opening between 

 the auricle and the ventricle, and this membrane, thus raised up all 

 round, closes the opening, and prevents the return of the principal 

 part of the blood that way, and it is urged through another opening 

 into the lungs. 



These fringes, which, in the dilated state of the ventricle, hang 

 loose, but which are forced up as the blood insinuates itself behind 

 them when the ventricle contracts, discharge the function of a perfect 

 valve : they are tied down to a certain extent by cords attached to 

 their edges, and which spring from certain fleshy or muscular columns 

 that arise within the ventricle. The edges, therefore, are permitted 

 to be elevated, until they have attained a horizontal direction, and 

 meet each other, and perfectly close the opening, and then are 

 stopped by these tendinous cords, which oppose their strength to the 

 further elevation of the fringes, and that regulated or increased by 

 the muscular power of the columns beneath. 



In the ox, the venous systen: is very large. The vessels are 



