102 



COMPARATIVE ZOOLOGY. 



pipes) take up any surplus, and return it to the blood. 

 At the same time, the venous part of the capillary net- 

 work absorbs the waste products of the tissues, expelling 

 the gases by the lungs, and the solid matters by the skin 

 and kidneys. The special function of the several constit- 

 uents of the blood is not wholly known. The colorless 

 corpuscles in Vertebrates are supposed to be the source of 

 the red disks. The latter are the carriers of oxygen, 

 which is taken up by their red matter (haemoglobin) in 

 the lungs, and given up to the tissues. The same office is 

 performed by the blue coloring- matter (hsemocyanin) in 

 the blood of certain Invertebrates, as the Squid and Lob- 

 ster. The carbon dioxide is taken up by the plasma. 



Like the solid tissues, the blood, which is in reality a 

 liquid tissue, is subject to waste and renewal, to growth 

 and decay. The loss is repaired from the products of 

 digestion, carried to the blood by the lacteals, or absorbed 

 directly by the capillaries of the digestive tract. The 

 white corpuscles are probably prepared in many parts of 

 the body, especially the liver, spleen, and lymphatic glands. 

 In the lower organisms, the nutritive food is prepared by 

 contact with the tissues, without passing through special 

 organs. Lymph differs from blood chiefly in containing 

 less albumen and fibrine, and no red disks. Chyle is 

 lymph loaded with fat globules, and is found in the lac- 

 teals and vessels connected with them during the absorp- 

 tion of food containing fat. 



