THE FROG 55 



Bile is in part an excretion, and conveys out of the body 

 certain waste materials derived from haemoglobin. Separately 

 it has practically no digestive action, though in the body it greatly 

 augments the action of the pancreatic juice, more particularly in 

 regard to its action in fat-splitting, and its salts help in emulsification. 



Pancreatic juice is a far more active digestive agent, and, 

 in addition to being alkaline, contains three principal ferments. 

 Trypsin is an enzyme splitting up the peptones to amino-acids, and 

 this has much the same action as pepsin in breaking down proteins, 

 save that it acts in an alkaline solution.* Amylase is a starch- 

 splitting enzyme that is more active than the pytalin of the saliva. 

 Lipase acts upon certain fats, breaking them down into glycerol and 

 fatty acids, which, in their turn, unite with the alkali present in the 

 pancreatic juice to form soap. The soap acts upon the remaining 

 fats in a mechanical way, aided also by the bile salts, and forms an 

 emulsion, i.e. a white milky-looking fluid, formed by minute fat 

 globules being suspended in a liquid. The result of these various 

 actions is that the food, having of coarse an alkaline reaction, is of 

 the consistency of milk, and is now termed chyle. 



During its passage through the remaining part of the 

 intestine ereptase continues splitting up the amino-acids, the various 

 digestive processes are completed, and the food is taken up or 

 absorbed by the walls. The colloids, now transformed into crystal- 

 loids, pass through the mucous membrane by osmosis into the under- 

 lying blood-vessels, and are thus distributed to various parts to be 

 utilised in the general anabolism. The fats, however finely emulsi- 

 fied, remain colloidal, and it appears as if the cells of the intestinal 

 wall actually take the small globules in and pass them out at their 

 inner end into special vessels, the lacteals. These vessels, with 

 similar ones in all parts of the body, form an auxiliary part of the 

 circulatory system known as the lymphatic system, and are specially 

 numerous in the intestinal walls. 



A certain amount of insoluble matter is always taken in 

 with the food, and this, together with the undigested residue, 

 accumulates in the rectum as more or less solid pellets, the faeces, 

 which are discharged as excreta from time to time. It is noticeable 

 when comparing an herbivorous am'mal, such as the rabbit, with a 

 carnivore, like the frog, that the intestine is a great deal longer 



* The action of trypsin is dependent on the presence of another substance, 

 enterokinase, which is produced by the mucous membrane of the duodenum 

 when it is acted upon by an acid . The very pancreatic fluid itself is not secreted 

 until a substance, secretin, is made by the mucosa of the duodenum and con- 

 veyed to the pancreas by the blood. Indeed, the digestive actions in this part 

 of the gut are too complex to be adequately dealt with here. 



