CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 3s 1 



canal. Towards the pyloric end, in what is sometimes called the 

 cmtrum pylori, the circular layer increases in thickness, and at the 

 pylorus is developed into a, thick ring called the sphincter of the 

 pylorus; a less marked circular sphincter is also present at the 

 cardiac orifice. 



The size of the cavity of the stomach varies from time to time 

 according to the bulk of contents present, and the condition of 

 the muscular fibres. When the stomach is empty, the muscular 

 fibres are in a state of tonic contraction, and the cavity is small ; 

 when the stomach is full, the muscular fibres though carrying out 

 as we shall see more or less rhythmical movements are as a whole 

 relaxed and extended, so that the cavity is large. The mucous 

 membrane in its natural condition so to speak is of such a size 

 that it forms a smooth even lining to the muscular coat when this 

 is extended and relaxed and the cavity of the stomach distended. 

 Hence when the stomach is empty, and the muscular coat con- 

 tracted, the mucous membrane is thrown into folds or rngw, which 

 on account of the preponderance of the circular muscular coat take 

 a longitudinal course, the loose submucous tissue allowing this 

 movement of the mucous over the muscular coat. 



The mucous membrane is relatively very thick, the thickness 

 being due to the fact that the membrane over its whole extent is 

 thickly studded with glands ; it may in fact be said to be almost 

 wholly composed of a number of short comparatively " simple " 

 glands placed vertically side by side and bound together by just 

 as much connective tissue as serves to carry the blood vessels and 

 lymphatics. These glands vary in size, shape and character in 

 different parts of the stomach, and the stomachs of different 

 animals present in these respects very considerable differences : 

 but, for present purposes, we may consider them as of two kinds, 

 the glands at the cardiac end of the stomach or " cardiac glands " 

 and the glands at the pyloric end, or "pyloric glands." 



$ 211. Cardiac glands. These are tubular glands, about 

 "5 mm. to 2 mm. in length by 50 /j, to 100 // in width, whose 

 course is not wholly straight but wavy or gently tortuous, and 

 frequently curved or bent at the blind end. Some are simple 

 or unbranched, but others divide into two, three or even more 

 tubes. They are packed together side by side in a vertical 

 position so closely that in sections of hardened and prepared 

 stomachs in which the blood vessels are for the most part emptied 

 of blood and the lymph spaces of lymph, each gland seems to be 

 separated from its neighbours by nothing more than an extremely 

 thin sheet of connective tissue seen in sections as almost a mere 

 line. In the living stomach when the numerous blood vessels in 

 this connective tissue are filled with blood, and the lymph spaces 

 are distended with lymph, the glands are separated from each 

 other by a considerable space equal probably to about their own 

 diameter. 



