150 PHYSIOLOGY FOR BEGINNERS CHAP. 



of the finely-divided fat, the lymph of these vessels, instead of 

 being colourless like the lymph in a limb for instance, is after 

 a meal, but only after a meal, milky white, and is called chyle. 

 The lymphatic vessels of the intestine, or lacteals, pass away 

 in the mesentery and conduct the chyle, as we have seen, to 

 the thoracic duct, from which it is poured into the blood at the 

 junction of the left jugular and subclavian veins. 



The tubular glands of the intestine secrete a small amount 

 of juice, but this is not of much importance for digestive 

 purposes. Cane-sugar taken as food and also the malt-sugar 

 formed by the action of saliva and pancreatic juice are changed, 

 into grape-sugar during the act of absorption, and pass into 

 the blood as grape-sugar. The intestinal juice probably helps 

 this change. As the peptone during absorption passes through 

 the epithelial cells of the villi and then through the walls of 

 the blood-vessels into the blood, it is changed into the special 

 proteids present in the blood. The portal vein therefore 

 carries to the liver blood containing water, salts, grape-sugar, 

 and proteids which it has gained from the alimentary canal. 



Movements of the Contents of the Intestine. The 

 contents are gradually moved along the intestine by the con- 

 traction of the muscular coat. When the muscle fibres of the 

 circular layer contract at any place the size of the intestine at 

 that place is diminished, and the contents are largely pressed 

 out of that part. The fibres just below this, then, take up the 

 contraction and press the contents farther on. In this way 

 the contraction passes along the intestine like a wave, always 

 travelling towards the large intestine. This kind of contrac- 

 tion is called peristaltic contraction. The longitudinal 

 muscular layer assists in the movement. By the time the 

 contents have reached the end of the small intestine much of 

 the material which can be of service to the body has been 

 absorbed. What is left, with the indigestible matter and the 

 remains of the juices, passes in a semi-fluid condition through 

 the ileocoecal valve into the large intestine 



Structure of the Large Intestine. The wall of tin- 

 large intestine consists, like that of the small intestine, of an 

 internal coat of mucous membrane, and an external coat of 

 plain muscle, consisting of two layers, an internal circular 

 and an external longitudinal. The muscular fibres of the 



