THE LIVER 95 



in the lining of the small intestine also secrete enzymes capable of 

 completing the action of those derived from the previous sources. 

 The actions of the dissolving enzymes are successive, secretion 

 being dependent to some extent on antecedent bodies by which the 

 stimulus for secretion is determined. The preliminary processes 

 of digestion refer in this way to the mechanical action of food 

 passage along the canal and to the provision of converting enzymes. 

 Absorption, which is the final object of the digestive process, is 

 accomplished in the lower part of the small intestine and in the 

 large intestine through the blood-vessels and lymphatics of the wall. 

 The relatively great extent of the wall, including the enormous 

 development of the caecum in the rabbit and other rodents, is 

 related to the comparatively great bulk and low nutritive quality 

 of the ingested food. The caecum also provides room for retention 

 of materials long enough for bacterial action upon cellulose, which 

 is not otherwise digested, to make resulting products available for 

 assimilation. 



The Liver 



The liver has a variety of other functions besides those men- 

 tioned above. It stores nutritive material in the form of glycogen 

 ("animal starch") and fat, and perhaps also protein, and plays an 

 important part in fat metabolism. It removes various waste 

 substances from the blood, eliminating some (bile pigments) in 

 the bile and preparing others, the nitrogen-containing substances 

 and toxic bodies absorbed from the colon, to be returned to the 

 circulation for final excretion by the kidneys. It is one of the 

 minor situations where red blood corpuscles are developed. From 

 it, as well as from various other tissues, is obtained a substance 

 (heparin) which prevents clotting of the blood. In the liver also 

 there is stored an antianaemic substance, formed by the action of 

 a specialized digestive enzyme on food protein, which stimulates 

 the production of red corpuscles in bone marrow. 



The liver is primarily a compound tubular gland, but during 

 development it becomes associated with the vascular system in the 

 formation of a structure quite peculiar. It is composed of numerous 

 minute units, the liver lobules, indications of which may often be 

 distinguished on the surface of a fresh or well-preserved liver. 



