106 HUMAN PHYSIOLOGY. 



of the gland lies a lymph-channel traversed by a reticulum of adenoid tissue. 

 The lymph-vessels, after penetrating this capsule, pour their lymph into this 

 channel, through which it passes; it is then collected by the efferent vessels 

 and transmitted onward. The lymph-corpuscles which are washed out of 

 of the gland into the lymph-stream are formed, most probably, by division 

 of preexisting cells. 



The thoracic duct is the general trunk of the lymphatic system; into it the 

 vessels of the lower extremities, of the abdominal organs, of the left side of 

 the head, and of the left arm empty their contents. It is about twenty inches 

 in length, arises in the abdomen, opposite the third lumbar vertebra, by a 

 dilatation (the receptaculum chyli}, ascends along the vertebral column to the 

 seventh cervical vertebra, and terminates in the venous system at the junction 

 of the internal jugular and subclavian veins on the left side. The lymphatics 

 of the right side of the head, of the right arm, and of the right side of the 

 thorax terminate in the right thoracic duct, about one inch in length, which 

 joins the venous system at the junction of the internal jugular and subclavian 

 on the right side. 



-The general arrangement of the lymph vessels is shown in figure 15. 



The blood-vessels which are concerned in the conduction of fresh nutri- 

 tive material from the alimentary canal have their origin in the elaborate 

 capillary network in the mucous membrane. The small veins which emerge 

 from the network gradually unite, forming larger and larger trunks, which are 

 known as the gastric, superior, and inferior mesenteric veins. These finally 

 unite to form the portal vein, a short trunk about three inches in length. 

 The portal vein enters the liver at the transverse fissure, after which it forms 

 a fine capillary plexus ramifying throughout the substance of the liver; from 

 this plexus the hepatic veins take their origin, and finally empty the blood 

 into the vena cava inferior. (See Fig. 16.) 



Absorption of Food. Physiological experiments have demonstrated that 

 the agents concerned in the absorption of new materials from the alimentary 

 canal are: 



1. The blood-vessels of the entire canal, but more particularly those uniting 

 to form the portal vein. 



2. The lymph vessels coming from the small intestine, which converge to 

 empty into the thoracic duct. 



As a result of the action of the digestive fluids upon the different classes 

 of food principles proteins, sugars, starches, and fats there are formed 

 amino- acids, dextrose and levulose, soap and glycerin, which differ from 

 the former in being highly diffusible a condition essential to their absorption. 



