158 PRACTICAL PHYSIOLOGY 



CHAPTER XLV. 

 ABSORPTION. 



Absorption of Fat. A cat is fed on milk, and three or four hours 

 later is killed. Open the abdomen and expose the mesentery. The 

 lacteals will appear as white cords, and the mesenteric lymphatic 

 glands turgid. Expose the thoracic duct at its entrance at the junction 

 of the left jugular and subclavian veins. Open the duct ; a milky white 

 fluid will escape. Fix some small pieces of jejunum in osmic acid, 1 per 

 cent, sol., and make teased preparations of the columnar cells in dilute 

 glycerine. The cells will appear full of fat droplets stained black. 



Absorption (Reid's Experiment). The abdomen is opened in the 

 anaesthetised animal. Two loops of small intestine are chosen of equal 

 length and ligated at either end. One loop, A, is washed out with 

 isotonic NaCl solution, the other, B, with distilled water or isotonic 

 saline + O1 per cent, sodic fluoride. 



An equal measured amount of the animal's own serum is then 

 placed in each loop. The serum is obtained by drawing off some 

 blood from the carotid artery. The blood is whipped and centrifuged. 

 After one hour 50 per cent, of the water may be absorbed from A and 

 none from B, in which the columnar epithelium is destroyed by the 

 reagents used to wash it out. 



Ten minutes' anaemia induced by clamping the mesenteric arteries has 

 the same effect in preventing absorption. The water, organic solids 

 and salts, are taken up by the intestinal wall from the animal's own 

 serum which has the same osmotic pressure as the blood in the mesen- 

 teric vessels. This occurs when the hydrostatic pressure of the gut is 

 below that of the blood in the mesenteric veins, and when the lacteals 

 of the loop are tied. Osmosis and filtration are thus excluded in this 

 experiment, and absorption must be ascribed to the selective activity 

 of the living columnar epithelium. 



