MULTICELLULAR ANIMALS 177 



tion within the cells is also carried away by the blood in chemical 

 combination with blood substances, including ha;moglobin, and not 

 primarily as merely a solution of the gas in the liquid portion of 

 the blood. The process of carbon dioxide conveyance is somewhat 

 more obscure than that of oxygen; it is known that in this process 

 too the haemoglobin plays a role, as do other blood substances, par- 

 ticularly NaHCOa, sodium bicarbonate. 



Lymph. In many animals lower than the vertebrates the cir- 

 culation of fluids within the body is unorganized, taking place in 

 and among the tissues without being confined within a definite 

 tubular system. A similar unorganized or open liquid circulation 

 occurs in vertebrates; since the blood is always confined within 

 the blood vessels, some medium is necessary to convey foods to, and 

 wastes away from the tissue cells and into and out of the blood 

 vessels. Earlier it was pointed out that all active animal cells are 

 aquatic; that is, each is surrounded by a watery liquid. This liquid 

 in the more complex animals is called the lymph; it serves as the 

 solvent which bathes the cells and in turn the walls of the fine 

 tubules of the blood system. Lymph is substantially equivalent to 

 the blood plasma minus the blood cells and clotting mechanism. 

 Through the walls of the blood vessels materials pass by diffusion 

 into the lymph and from the lymph into the tissue cells. The lymph 

 is not in general enclosed within tubular walls. However, small 

 lymph vessels do serve to collect this liquid draining through the 

 tissues and to return it to the veins. Chief among these lymph 

 vessels in the human body is the thoracic duct, which receives all 

 the lymph that is returned into the blood vessels and empties into 

 the great vein near the heart (Fig. 123). It also receives the digested 

 fats derived from the small lymph vessels, lacteals, that are dis- 

 tributed in the wall of the intestine, so that fats are not passed 

 through the liver after digestion but are short-circuited, so to speak, 

 and taken directly to the main circulation. The other products of 



