274 LECTURES TO SCIENCE TEACHERS. 



By their action we get the albuminous constituents of the 

 food converted into what are called peptones, soluble kinds 

 of albuminous substances which dift'er very considerably 

 from the other forms. We get starch converted into grape 

 sugar, and both this and peptones diffuse readily through 

 animal membranes, although albumin and starch -do not. 



In the collection we have a piece of apparatus which not 

 only dissolves the food in the way I have shown you, but 

 also shows you the passage of the food after solution 

 through animal membranes. It consists of a truncated 

 funnel, containing a filter made of a piece of bladder, and 

 placed in a jar of pure water. Into the filter are put some 

 fibrine, pepsin, and dilute hydrochloric acid, and the whole 

 is then put in a water-bath at blood-heat. As the fibrine is 

 digested, the peptones which are formed diffuse through the 

 bladder into the water, and thus we get a good imitation of 

 digestion in the stomach, where peptones are removed by 

 absorption at the same time that more are being formed by 

 the action of the gastric juice (No. 3496 in Catalogue}. 



There is only one other apparatus which I shall describe 

 to you. Here you have a sheep which is kept constantly 

 in one position. It may be fed regularly, and all the pro- 

 ducts of respiration may be collected if you like by 

 putting the animal into a large Pettenkofer's apparatus. 

 But the products of respiration form only one part of the 

 products of the decomposition of tissues : a large part also 

 passes out in the urine. Food contains a quantity of 

 nitrogenous substances, and the waste products to which 

 these give rise, after they have fulfilled their purpose in 

 the body, pass out of it in the urine, and this must all be 

 collected in order to ascertain what the decomposition of 

 the tissues in an animal has been, The apparatus consists 

 simply of a large iron case, in which the animal is kept 

 fixed by straps, and in the vessel below the urine can be 

 collected. The food can be collected as it is put in ; and 

 any food that is not consumed may be estimated \ so that 

 you can ascertain exactly the food that has been taken in 

 a given time, the quantity of carbonic acid given off and 

 the quantity of oxygen used ; and. you can collect under- 

 neath the urine, and estimate the quantity of nitrogenous 

 waste given out in a given time. In this way you can 



