GENERAL PLAN OF THE CIRCULATION 



459 



sex. According to Messrs. Prevost and Dumas, the blood of the horse 

 contains less solid matter than that of man, in the proportion of 9"20 to 12'92 

 in 1000 parts. The temperature is about 99 '5 degrees of Fahrenheit's 

 thermometer ; the pulse vai'ies greatly according to the breed of horse, being 

 as slow as 32 to the minute in some heavy cart-horses, and as high as 

 42 in high-bred and sensitive specimens of the I'ace. The shade of colour 

 in the red coi'puscles depends upon the proportion of carbonic acid and 

 oxygen combined with them. If the former preponderates, a deep purple- 

 red is developed, known as that of venous blood : while a liberal supply of 

 oxygen develops the bright scarlet peculiar to arterial blood. The saline 

 matters dissolved in the liquor sanguinis consist chiefly of the chlorides of 

 sodium and potassium (which comprise more than one-half of the whole 

 salts), the tribasic phosphate of soda, the phosphates of magnesia and lime, 

 sulphate of soda, and a little of the phosphate and oxide of iron. 



GENERAL PLAN OF THE CIRCULATION 



The blood is circulated through the body, for the purposes of nutrition 

 and secretion, by means of one forcing pump, and through the lungs, for its 

 proper aeration, by another ; the two being united to form the heart. This 

 organ is therefore a compound machine, 

 though the two pumps are joined together, 

 so as to appear to the casual observer to 

 be one single organ. In common lan- 

 guage, the heart of the mammalia is saitl 

 to have two sides, each of which is a 

 forcing pump ; but the blood, before it 

 passes fi'om one side to the other, has to 

 circulate through one or other of the sets 

 of vessels found in the general organs of 

 the body, and in the lungs, as the case 

 may be. This is shown at Fig. 70, where 

 the blood, commencing with the capillaries 

 on the general surface at A, passes through 

 the veins which finally end in the vena 

 cava (B), and enters the right auricle (C). 

 From this it is pumped into the right 

 ventricle (D), which, contracting in its 

 turn, forces it on into the pulmonary 

 artery (E), spreading out upon the lining 

 membrane of the lungs, to form the capil- 

 laries of that organ at F, from which it is 

 returned to the left auricle (G) through 

 the pulmonary veins. From the left auricle 

 it is driven on to the left ventricle ; and 

 this, by its powerful contractions, forces the blood through the aorta (I), and 

 the arteries of the whole body, to the capillaries (A), from which the descrip- 

 tion commenced. But though this organ is thus made up of two pumps, yet 

 they are united into one organ, and the two auricles and two ventricles each 



Fig. 70.— Plan of the Circui. 



ATION. 



A. Caiiillaries on the general surface. 



B. Ven.a cava. 



0. Right auricle. 



D. Right ventricle. 



E. Pulmonary artery. 



P. Cajiillaries of the lungs, uniting to form 



the iiulmonary veins, which enter 

 G. The left auricle. 

 H. The left ventricle. 



1. The aorta posterior, dividing into smaller 



arteries, and united with the capilla- 

 ries at A. 

 J. The trunk of the aorta anterior. 



