THE DIGESTIVE JUICES 



99 



Absorption of fats. The fats undergo in the intestine two 

 changes : one a physical change (emulsification), the other a chemical 

 change (saponification). The lymphatic vessels are the great channels 

 for fat-absorption, and their name, lacteals, is derived from the 

 milk-like appearance of their contents (chyle) during the absorption 

 of fat. 



The way in which the minute fat globules pass from the intestine 

 into the lacteals has been studied by killing animals at varying periods 

 after a meal of fat and making osmic acid microscopic preparations 

 of the villi. Figs. 26 and 27 illustrate the appearances observed. 



The columnar epithelium cells become first filled with fatty 

 globules of varying size, which are generally larger near the free 

 border. The globules pass down the cells, the larger ones breaking 

 up into smaller ones during the journey ; they are then transferred 

 to the amoeboid cells of the lymphoid 

 tissue beneath : these ultimately pene- 

 trate into the central lacteal, where 

 they either disintegrate or discharge 

 their cargo into the lymph stream. 

 The globules are by this time divided 

 into immeasurably small ones, the 

 molecular basis of chyle. The chyle 

 enters the blood stream by the thoracic 

 duct, and after an abundant fatty meal 

 the blood plasma is quite milky; the 

 fat droplets are so small that they 

 circulate without hindrance through 



the capillaries. The fat in the blood after a meal is eventually stored 

 up in connective tissue cells of adipose tissue. It must, however, 

 be borne in mind that the fat of the body is not exclusively derived 

 from the fat of the food, but it may originate also from carbo- 

 hydrates, and possibly, in the opinion of some physiologists, from 

 protein as well. 



As the fat globules were never seen penetrating the striated 

 border of the epithelial cells, there was a difficulty in understanding 

 how they reached the interior of these cells ; the cells will not take 

 up other particles, and it is certain that they do not in the higher 

 animals protrude pseudopodia from their borders (this, however, does 

 occur in the endoderm of some of the lower invertebrates).^ 



Recent research has solved this difficulty;, Li i;he rst: place 

 particles may be present in the epithelium' arncl lyrnphoid cells while 

 no fat is being absorbed. These particles arypiOuOpla3mic in nature, 





