THE ALIMENTARY CANAL AND ITS APPENDAGES. 131 



of the saliva. Finding that the epithelial cells of the stomach 

 were often loaded with oil-drops, he concluded that absorption, 

 at least of fats, takes place here. The chylific stomach, care- 

 fully emptied of its contents, was found to convert starch into 

 sugar at ordinary temperatures. The saliva of the Cockroach 

 gave a similar result, and when a weak solution of hydrochloric 

 acid was added, Basch thought that the mixture could digest 

 blood-fibrin at ordinary temperatures. 



Plateau's researches upon Periplaneta americana* modified by 

 subsequent experiments upon P. orientalist and by still more 

 recent observations, lead him to the following conclusion S+ : 



1. The saliva of the Cockroach changes starch into glucose; 

 but the saliva is not acid, it is either neutral (P. orientalis) or 

 alkaline (P. americana). Any decided acidity found in the crop 

 is due to the ingestion of acid food ; but a very faint acidity 

 may occur, which results from the presence in the crop of a 

 fluid secreted by the caecal diverticula of the mesenteron. 



2. The glucose thus formed is absorbed in the crop, and no 

 more is formed in the succeeding parts of the digestive tube. 



3. The function of the gizzard is that of a grating or 

 strainer. It has no power of trituration. If the animal con- 

 sumes vegetable food rich in cellulose, a substance not' capable 

 of digestion in the crop, the fragments are found ' unaltered as 

 to form and size in the mesenteron. If it is supplied with 

 plenty of farinaceous food, such as meal or flour, the saliva is 

 not adequate to the complete solution and transformation of the 

 starch, and the intestine is found full of uninjured starch 

 granules, which must have traversed the gizzard without 



<*. / * O 



crushing. 



4. The coecal diverticula secrete a feebly acid fluid. To 

 demonstrate its acidity an extremely sensitive litmus solution, 

 capable of indicating one part in twenty thousand of hydro- 

 chloric acid, must be used. The fluid secreted by the caeca 

 emulsifies fats, and converts albuminoids into peptones. 



In all Insects digestion is effected in the following way 

 (which is particularly easy of demonstration in Carabus and 



* Bull. Acad. Eoy. de Belgique, 1876. 

 t Ib.,1877. 



+ We are indebted to Prof. Plateau for the statement of his views given in the 

 text. 



