CIRCULATORY SYSTEM. 



PRELIMINARY REMARKS (ON THE BLOOD, 

 ETC.). 



The appearance of blood is familiar to 

 most persons. It contains the elements for 

 building up and nourishing the whole ani- 

 mal structure. On examining blood with a 

 microscope, it is found full of little red glo- 

 bules, which vary in their size and shape in 

 different animals, and are more numerous 

 in warm than in cold-blooded animals ; 

 probably this arises from the fact that the 

 latter absorb less oxygen. If the blood of 

 one animal be transfused into another, it 

 will frequently cause death. 



When blood stands for a time after being 

 drawn, it separates into two parts. One is 

 called serum, and resembles the white of an 

 egg ; the other is the clot or crassamentum, 

 and forms the red coagulum, or jelly-like 

 substance : this is accompanied by whitish, 

 tough threads, called fibrine. When blood 

 has been drawn from a horse, and it as- 

 sumes a cupped or hollow form, if serum, 

 or buify coat, remain on its surface, it de- 

 notes an impoverished state ; but if the 

 whole, when coagulated, be of one uniform 

 mass, it indicates a healthy state of this 

 fluid. The blood of a young horse gen- 

 erally coagulates into a firm mass, while 

 that of an old or debilitated one is gen- 

 erally less dense, and more easily divided or 

 broken down. The power that propels the 

 blood into the different ramifications of the 

 animal, is a mechanico-vital power, and is 

 accomplished through the medium of the 

 heart and lungs ; the latter is a powerful 

 muscular organ contained in the chest. 

 From certain parts of it arteries arise ; in 

 others the veins terminate ; and it is princi- 

 pally by its alternate contractions and ex- 

 pansions, aided as already stated, that the 

 circulation of the blood is carried on. The 



heart is invested with a membranous sac, 

 called pericardium, which adheres to the 

 tendinous centre of the diaphragm, and to 

 the great vessels at the base of the heart. 

 The heart is lubricated by a serous fluid 

 within the pericardium, which guards 

 against friction. In dropsical affections, 

 the quantity of this fluid is considerably in- 

 creased, and constitutes a disease called hy- 

 drothorax. The heart is divided into four 

 cavities, viz., two auricles, named from 

 their resemblance to an ear, and two ventri- 

 cles, forming the body. The left ventricle 

 is smaller than the right ; but its sides are 

 much thicker and stronger : it is from this 

 part that the grand trunk of the arteries 

 proceeds, called the great aorta. The right 

 cavity, or ventricle, is the receptacle for the 

 blood that is brought back by the veins 

 after going the rounds of the circulation ; 

 which, like an inverted tree, become larger 

 and less numerous as they approach the 

 heart, where they terminate in the right 

 auricle. The auricle on the left side of 

 the heart receives the blood that has been 

 distributed through the lungs for purifica- 

 cation. Where the veins terminate in auri- 

 cles, there are valves placed. The coronary 

 vein, which enters the right auricle, has its 

 mouth protected by a valve called semi- 

 lunar, or half-moon shape, which opens only 

 toward the heart, and prevents the blood 

 taking a retrograde course. The different 

 tubes coming from and entering into the 

 heart are also provided with valves to pre- 

 vent the blood from returning. For exam- 

 ple, the blood proceeds out of the heart, 

 along the aorta; the valve opens forward or 

 upward, the blood also moves upward, and 

 pushes the valve asunder, and passes 

 through ; the pressure from above effec- 

 tually closes the passage. The valves of 



(155) 



