252 THE TISSUES AND MECHANISMS OF DIGESTION. 



taken the stomach is at rest and no secretion takes place. When food is 

 taken, the characters of the gastric juice secreted are obscured by the food 

 with which it is mingled. The gastric membrane may, it is true, be arti- 

 ficially stimulated, by touch, for instance, and a secretion obtained. This 

 we may speak of as gastric juice, but it may be doubted whether it ought 

 to be considered as normal gastric juice. And indeed, as we shall see, even 

 the juice which is poured into the stomach during a meal, varies in compo- 

 sition as digestion is going on. Hence the characters which we shall give 

 of gastric juice must be considered as having a general value only. 



Gastric juice, obtained in as normal a condition as possible from the 

 healthy stomach of a fasting dog by means of a gastric fistula, is a thin, 

 almost colorless fluid with a sour taste and odor. 



In the operation for gastric fistula an incision is made through the abdominal 

 walls, along the linea alba, the stomach is opened, and the lips of the gastric 

 wound securely sewn to those of the incision in the abdominal walls. TJnion 

 soon takes place, so that a permanent opening from the exterior into the inside 

 of the stomach is established. A tube of proper construction, introduced at the 

 time of the operation, becomes firmly secured in place by the contraction of 

 healing. Through the tube the contents of the stomach can be received, and the 

 mucous membrane stimulated at pleasure. 



When obtained from a natural fistula in man, its specific gravity has 

 been found to diflfer little from that of water, varying from 1.001 to 1.010, 

 and the amount of solids present to be correspondingly small. In animals 

 pure gastric juice seems to be equally poor in solids, the higher estimates 

 which some observers have obtained being probably due to admixture with 

 food, etc. 



Of the solid matters present about half are inorganic salts, chiefly alka- 

 line (sodium) chlorides, with small quantities of phosphates. The organic 

 material consists of pepsin, a body to be described immediately, mixed with 

 other substances of undetermined nature. In a healthy stomach gastric juice 

 contains a very small quantity only of mucin, unless some submaxillary 

 saliva has been swallowed. 



The reaction is distinctly acid, and the acidity is normally due to free 

 hydrochloric acid. This is shown by various proofs, among which we may 

 mention the conclusive fact that the amount of chlorine present in gastric 

 juice is more than would suffice to form chlorides with all the bases present, 

 and that the excess, if regarded as existing in the form of hydrochloric acid, 

 corresponds exactly to the quantity of free acid present. Lactic and butyric 

 and other acids when present are secondary products, arising either by their 

 respective fermentations from articles of food, or from the decomposition of 

 their alkaline or other salts. In man the amount of free hydrochloric acid 

 in healthy juice may be stated to be about 0.2 per cent., but in some animals 

 it is probably higher. 



188. On starch gastric juice has no amylolytic action ; on the con- 

 trary, when saliva is mixed with gastric juice any amylolytic ferment which 

 may be present in the former is at once prevented from acting by the acidity 

 of the mixture. Moreover, in a very short time, especially at the tempera- 

 ture of the body, the amylolytic ferment is destroyed by the acid, so that 

 even on neutralization the mixture is unable to convert starch into sugar. 



On dextrose healthy gastric juice has no effect. And its power of con- 

 verting cane-sugar seems to be less than that of hydrochloric acid diluted to 

 the same degree of acidity as itself. In an unhealthy stomach, however, 

 containing much mucus, the gastric juice is very active in converting cane- 

 sugar into dextrose. This power seems to be due to the presence in the 

 mucus of a special ferment, analogous to, but quite distinct from, the 



