THE CIRCULATION OF THE BLOOD. 177 



called the blood corpuscles. From the blood the tissues of the 

 body derive materials for their nutrition, growth, or repair, and 

 for their secretions. The blood is replenished with new matter 

 from the digestive organs, which dissolve and prepare the food 

 for this purpose. To carry on circulation, an apparatus is pro- 

 vided which consists of the heart, the arteries, the capillaries which 

 form the connecting link between the extremities of the smallest 

 branches of the arteries and those of the veins; the veins; the ar- 

 teries of the lungs ; the lungs with their capillaries, and the veins of 

 the lungs. There is thus a double circulation as it were, which 

 may be readily understood by a reference to the diagram, fig. 69, 



in which C I) represents 

 the left auricle of the 

 heart, which forces the 

 blood through the arteries 

 E, in the direction of the 

 arrows to the fine net- 

 work of the capillaries; 

 then to the veins F, and 

 Fig. 69. DIAGRAM OF THE oiEOULATioN. thence to the right auricle 



of the heart, A B. From 



this it is forced through the right ventricle which opens from the 

 right auricle into the pulmonary artery G, still following the course 

 indicated by the arrows, into the net-work of the capillaries of the 

 lungs, from which it is conveyed by the pulmonary vein H t into 

 the left ventricle, whence it passes to the left auricle, on the same 

 course over again. The heart is a mass of very strong muscular 

 fiber, having the four cavities just mentioned, and being supplied 

 with valves which regulate the flow of the blood. The muscles of 

 the heart contract and expand with regularity, performing what 

 we usually term its " beats," four times or thereabouts for every 

 inspiration of the lungs. In a young sheep the heart beats 80 to 

 90 times in a minute ; a full grown one, 70 to 80 times, and in a 

 very old one, 55 to 60 times. At each contraction the blood is 

 forced through the arteries and their branches to the capillaries. 

 These capillaries are exceedingly small, being from y 20 oo to Yseooth 

 part of an inch hi diameter, and inosculate or join together again 

 and again, forming a net-work of the closest character, so close 

 that the finest needle cannot penetrate the skin or membranes any- 

 where without wounding one or more of them, and causing an 

 escape of blood. While circulating in this net- work of capillaries, 

 the blood gives up to the tissues amongst which it circulates, the 

 materials needed for their growth and increase, and also to sup- 



