306 COMPARATIVE ZOOLOGY 



a gallon and a half, equal to one-thirteenth of his 

 weight. The heart of the Greenland whale is a yard 

 in diameter. 



The main Office of the Blood is to supply nourishment 

 to, and take away waste matters from, all parts of the 

 body. It is at once purveyor and scavenger. In its 

 circulation, it passes, while in the capillaries, within an 

 infinitesimal distance of the various tissue cells. Some of 

 the plasma, carrying the nutritive matter needed, exudes 

 through the walls of the capillary tubes ; the tissue as- 

 similates or makes like to itself whatever is suitable for 

 its growth and repair ; and the lymphatics take up the 

 transuded fluid, and return it to the blood vessels. At 

 the same time, the waste products of the tissues are col- 

 lected and brought through the venous capillaries, veins, 

 and lymphatics to the excretory organs. The special 

 function of the several constituents of the blood is not 

 wholly known. The corpuscles in the red marrow of 

 the bones of some 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 (hemo- 

 globin) in the lungs and given up to the tissues. The 

 same office is performed by the blue coloring matter 

 (haemocyanin) in the blood of certain invertebrates, as 

 the squid and lobster. The carbon dioxide is taken up 

 mainly 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 ab- 

 sorbed directly by the capillaries of the digestive tract. 

 The white corpuscles probably are prepared in many 

 parts of the body, especially the liver, spleen, and lym- 

 phatic glands. In the lower organisms, the nutritive 

 food is prepared by contact with the tissues, without 



