638 PHYSIOLOGY 



collagen, converting it, probably through the stage of gelatin, into gelatoses 

 and gelatin peptones, bearing the same relation to the original substance 

 as is borne by the proteoses and peptones to the proteins. On account of 

 this action, adipose tissue (which consists of protoplasmic cells distended with 

 fat, and bound together by connective tissue) is broken up into its constituent 

 cells. The protoplasmic pellicle is dissolved, and the fat floats freely in the 

 gastric juice. 



ELASTIN, which also occurs in varying amounts as the chief constituent 

 of the elastic fibres of connective tissues, is slowly acted upon by gastric 

 juice. Under the conditions of natural digestion, however, it may be 

 regarded as indigestible. 



MUCIN, which forms a considerable proportion of the ground substance 

 of connective tissues, is converted by gastric juice into peptone-like sub- 

 stances, and into reducing bodies probably allied to glycosamine. 



The NUCLEO -PROTEINS, the chief constituents of cells, and therefore in- 

 gested in large amounts with food-stuffs such as sweetbreads, are first 

 dissolved by the acid of the gastric juice, and are then broken up into two 

 moieties. The protein half is converted into proteoses and peptones, while 

 the nuclein moiety is precipitated in an insoluble form. 



On PHOSPHO-PROTEINS gastric juice acts in a somewhat similar manner. 

 The protein of milk, caseinogen, undergoes special changes in the stomach. 

 The first effect of gastric juice, even in neutral medium, is to convert the 

 caseinogen into an insoluble casein. This action is generally ascribed to the 

 presence of a distinct ferment of the gastric juice, named rennin, or rennet 

 ferment. But, according to some authorities, it is due directly to the pepsin, 

 i.e. rennin and pepsin are identical. For the conversion of caseinogen into 

 the solid clot of casein the presence of lime salts is necessary. The addition 

 of rennet to an oxalated milk apparently produces no effect, but clotting 

 ensues if a soluble lime salt, such as calcium chloride, is then added to the 

 mixture. Under the action of the acid gastric juice the solid clot of casein 

 is dissolved, but a precipitate is left containing a small proportion of the 

 original phosphorus of the caseinogen. This precipitate is sometimes 

 spoken of as para-nuclein, or pseudo-nuclein. It does not yield purine 

 bases on hydrolysis with acids, but contains phosphoric acid in organic com- 

 bination. By prolonged digestion with strong gastric juice it is possible to 

 dissolve the whole of this precipitate. It is therefore thought that in the 

 clotting of milk the caseinogen under the action of the rennet first undergoes 

 a conversion into a soluble casein, or perhaps a splitting into a soluble casein 

 and some other protein. The soluble casein then, under the influence of 

 the lime salts, forms an insoluble casein which is precipitated, and causes the 

 solidification of the milk. In the absence of lime salts, the conversion or 

 splitting of caseinogen takes place, but the second stage of the process cannot 

 occur until the lime salts are added. 



