CHAP. iv. CIRCULATION OF THE BLOOD. 275 



intestine are seen to contain a milky-looking fluid termed chyle, 1 that 

 these tubes have received the name of lacteals. It may be well to 

 mention here that this fluid is quite different and distinct from that 

 which is obtained from the mammary gland. The lacteals which 

 contain it find their way to the posterior end of a long irregular tube, 

 which extends beneath the spine, and there discharge their contents. 

 Because the greater part of the length of this tube extends along the 

 dorsal side of the thorax, or chest, it is termed the thoracic duct, and 

 the dilated posterior end which receives the emulsion of fat from the 

 lacteals is appropriately named the receptacle of the chyle. The thoracic 

 duct also receives the fluid collected by most of the lymphatics from all 

 parts of the body, but this is a thin watery fluid consisting chiefly of 

 the oozings through the walls of the blood capillaries. The thoracic 

 duct must, of course, get rid of the materials it is continually receiving, 

 and in the cow it pours its contents, about in the region of the first rib, 

 into a large vein which joins almost immediately the anterior vena 

 cava, another great vein opening into the right side of the heart. 

 Thus it appears that the nutritious ingredients of the food, whether 

 they leave the intestine by the blood\ capillaries of the villi, or by the 

 lacteal roots of the same structures, lind their way into the blood of 

 the right side of the heart. 



The tubes or vessels which convey blood to the heart are called 

 veins, those which carry blood from the heart are arteries. The heart 

 is a hollow muscle possessing two main cavities, one on each side, and 

 so arranged that there is no lateral communication in the heart itself 

 between these two chambers. The entire apparatus of the circulation 

 is so constructed that the rhythmical contractions of the heart shall 

 drive the blood in one direction, and in one direction only. When the 

 heart contracts and its contraction produces the " beat" the blood 

 is driven into certain arteries which break up into smaller arteries, and 

 finally into exceedingly narrow tubes called capillaries, so that if a fleshy 

 part of the body be cut without injuring any blood vessel discernible by 

 the unaided eye, the blood nevertheless wells forth from the severed 

 capillaries. The capillaries gradually coalesce into small veins, and 

 these into large veins, while the largest veins of all pour their blood 

 into the heart again. The blood on the right side of the heart differs 

 from that on the left side ; the former is dark, almost black, while the 

 latter is bright scarlet. The reason for this may be discovered by 

 examining somewhat more carefully the results of the heart's action. 



Commencing with the blood in the right side of the heart, the effect 

 of the systole or contraction of the organ is to drive the dark blood 

 out through the pulmonary artery into the lungs, in the capillaries of 

 which the hot dark fluid is exposed to the influence of atmospheric air. 

 The blood receives from the air its oxygen gas, and gives in exchange 

 carbonic acid gas and water vapour, which pass out in the expired air. 

 Tt is this deprivation of carbonic acid gas and addition of oxygen gas which 

 causes the dark blood to become scarlet. The blood in the capillaries 



1 Gr. chulos, juice. 



T 2 



