DIGESTION. 245 



of the tubes, and form the framework on which are moulded the small 

 elevated ridges of mucous membrane bounding the minute, polygonal 

 pits before referred to. From this superficial network the veins chiefly 

 take their origin. Thence passing down between the tubes, with no very 

 free connection with the deeper inter-tubular capillary plexus, they open 

 finally into the venous network in the submucous tissue. 



Nerves. The nerves of the stomach are derived from the pneu mo- 

 gastric and sympathetic, and form a plexus in the submucous and mus- 

 cular coats, containing many ganglia (Remak, Meissner). 



DIGESTION IN THE STOMACH. 



Gastric Juice. The functions of the stomach are to secrete a diges- 

 tive fluid (gastric' juice), to the action of which the food is next subjected 

 after it has entered the cavity of the stomach from the oesophagus; to 

 thoroughly incorporate the fluid with the food by means of its muscular 

 movements; and to absorb such substances as are capable of absorption. 

 While the stomach contains no food, and is inactive, no gastric fluid is 

 secreted; and mucus, which is either neutral or slightly alkaline, covers 

 its surface. But immediately on the introduction -of food or other sub- 

 stance the mucous membrane, previously quite pale, becomes slightly 

 turgid and reddened with the influx of a larger quantity of blood; the 

 gastric glands commence secreting actively, and an acid fluid is poured 

 out in minute drops, which gradually run together and flow down the 

 walls of the stomach, or soak into the substances within it. 



Chemical Composition of Gastric Juice. The first accurate 

 analysis of gastric juice was made by Prout: but it does not appear to 

 have been collected in any large quantity, or pure and separate from food, 

 until the time when Beaumont was enabled, by a fortunate circumstance, 

 to obtain it from the stomach of a man named St. Martin, in whom there 

 existed, as the result of a gunshot wound, an opening leading directly 

 into the stomach, near the upper extremity of the great curvature, and 

 three inches from the cardiac orifice. The introduction of any mechanical 

 irritant, such as the bulb of a thermometer, into the stomach, excited at 

 once the secretion of gastric fluid. This was drawn off, and was often 

 obtained to the extent of nearly an ounce. The introduction of aliment- 

 ary substances caused a much more rapid and abundant secretion than 

 did other mechanical irritants. Xo increase of temperature could be 

 detected during the most active secretion; the thermometer introduced 

 into the stomach always stood at 100 F. (37*8 C.) except during muscu- 

 lar exertion, when the temperature of the stomach, like that of other 

 parts of the body, rose one or two degrees higher. 



The chemical composition of human gastric juice has been also in- 

 vestigated by Schmidt. The fluid in this case was obtained by means of an 



