THE STUDY OF ORGANS. 45 



blood-vessels ; but instead of ending, like the blood- 

 vessels, in fine hair-like branches, called capillaries, 

 they end in irregular spaces. The division of the 

 nutritive fluid system into two kinds, lymph and blood, 

 is peculiar to the vertebrates. The glands and vessels 

 are in close connection with the villi of the intestine, 

 by branches called lacteals, through which they receive 

 from the villi the liquid products of digestion, ■which 

 the villi absorb from the inner surface of the alimentary 

 canal. Thus the lymph is replenished with liquid 

 from the alimentary canal, and with cells from the 

 lymphatic glands. All the small branches of the 

 lymphatic system join on the left side to form one 

 vessel, called the thoracic duct, because it lies in 

 the region of the thorax (chest), and there opens into 

 the internal jugular vein of the blood system. There 

 is a similar but much smaller and less important vessel 

 on the right side. The system of blood-vessels is 

 closed all but these openings, through which it is 

 replenished with fluid and with white corpuscles. 



The Blood is contained in a system of vessels which 

 in the higher animals have contractile walls, that 

 drive the contents along. The heart is in its simplest 

 form merely a tublilar contractile vessel, that becomes 

 in mammalia complicated into four chambers, two 

 auricles and two ventricles ; this structure is produced 

 by the tube twisting and doubling upon itself during 

 the course of its development. The vessels which take 

 the blood out of it are called arteries ; these pass to 

 all parts of the body and divide into capillaries (i.e. 



