814 



DIGESTION" 



previously were collapsed now become distended, 

 and a little wave of air at high pressure is forced 

 into the middle ear, causing a buzzing sound. 

 Occasionally the middle ear communicates with 

 the external air through congenital or acquired 

 apertures in the membrana tympani. In these 

 cases smoke may be propelled from the throat out 

 of one or both ears. 



The gullet or oasophagus (figs. 1, 2, and 3) is a 

 long tube passing from the pharynx to the stomach. 

 Its mucous coat is loaded with very large mucous 

 glands, which secrete a quantity of very viscid 

 mucus. Its muscular walls contain striped fibres 

 in the upper, unstriped in the lower part. The 

 stomach itself is a greatly dilated part of the 

 digestive system. Its shape is indicated in the fig. 

 It may be said to consist of two parts, even in the 

 human subject ; a more complex arrangement is 

 found in many animals, such as the ruminants. 

 The large dilated portion into which the gullet 

 opens is termed cardiac, and the opening the 

 cardiac or oesophageal opening. The narrow part 

 opening into the duodenum is the pyloric part, 

 and the opening the pyloric opening. The 

 whole is lined with mucous membrane, which, 

 in the empty stomach, is thrown into projecting 

 folds or rugae, but these folds are effaced when 

 the organ is distended with food (fig. 3). In the 



Fig. 3. Section of the Stomach : 



a, ducts of liver ; 6, pylorus ; c, bile duct ; d, pancreatic duct ; 

 e, cardiac orifice. 



membrane are innumerable glands, which secrete 

 the digestive juices of the stomach (fig. 4). If the 

 surface of the membrane be examined with a strong 

 pocket-lens, the apertures of these little glands may 

 be seen. They run down from the surface into the 

 deeper parts of the mucous membrane. They are 

 lined by secreting cells. The greater number of 

 glands situated in mucous membranes have the 

 simple structure diagrammatically represented in 

 fig. 4. Each gland has a mouth or short duct lined 

 by cells. Below this the little tube is lined by 

 cells which secrete the juice peculiar to the gland. 

 This secreting part sometimes branches. Outside 

 the gland blood-capillaries ramify, which supply 

 the gland with nourishment, enabling it to manu- 

 facture the substances which it secretes. The 

 gastric juice is acid, and the chief acid secreted 

 is hydrochloric acid. This is formed at the car- 

 diac, but not at the pyloric end. The substance 

 called pepsine, which is necessary for digestion, 

 is secreted by the whole of the glands. The 

 cardiac glands therefore secrete both substances, 

 and they possess two sorts of cells, those which 

 form the hydrochloric acid being bigger and more 

 granular than the other sort which secrete pepsine. 

 The pyloric glands secreting pepsine have onfy one 

 kind of cell similar to the pepsine-secreting cell of 

 the cardiac end. Outside the vascular and glandu- 

 lar mucous coat, and united to it by a loose delicate 



layer submucous coat is the muscular coat. Thie 



is similar to that of the rest of the alimentary 



canal, except that there are in addition to the 



circular and longitudinal fibres, many oblique 



fibres. The circular fibres are 



very thick indeed at the pyloric 



aperture, forming a circular 



sphincter band, which contracts 



and keeps back the food in the 



stomach until gastric digestion 



is nearly completed. 



The food, now called the 

 chyme, passes into the small 

 intestine, a tube some 20 feet 

 long. This tube, besides the 

 muscular and mucous coats, 

 possesses in addition an ex- 

 ternal coat of loose fibrous 

 tissue, covered by a single layer 

 of flat epithelial cells. This 

 coat is prolonged into, and 

 helps to form the mesentery, 

 a membrane connecting the 

 intestine with the abdominal 

 walls, which are lined with a 

 similar fibro-epithelial coat. 

 This membrane is called the 

 peritoneum, and is sometimes 

 inflamed ( peritonitis ) as a result 

 of cold, injuries, &c. The small 

 intestine is somewhat arbitrarily 

 divided into three portions the 

 upper (duodenum), the middle 

 (jejunum), and the lower 

 (ileum). In all parts the mus- 

 cular coat is similar to that of 

 the rest of the digestive system. 

 The mucous coat contains glands 

 very like the pyloric glands of 

 the stomach, called Lieber- 

 kiihn's follicles. These, however, rarely branch. 

 They secrete the intestinal juice. In the duo- 

 denum, one finds in addition highly-branched 

 glands called Brunner's. These extend right down 

 into the submucous coat. Little is known con- 

 cerning their function. In both the mucous and 

 submucous coats, and generally involving both 

 layers, are found masses of tissue lymphoid 

 similar to that found in a lymphatic gland (fig. 6). 

 These are termed solitary glands, but it must 

 be understood that they do not secrete any 

 juice concerned in digestion. Their function is 

 probably connected with the blood and the blood- 

 corpuscles. Collections of these solitary glands, 



d 



Fig. 4. 



B, cardiac gland from 

 the middle of the 

 human stomach, 

 magnified about 150 

 diameters : a, wall 

 of the tube, lined 

 with large oval nuc- 

 leated cells ; 6, the 

 same cells isolated ; 

 c, nucleated cells of 

 columnar epithe- 

 lium, occupying the 

 upper parts of the 

 tubes; d, blind ex- 

 tremity of the tube. 



Fig. 5. The under surface of the Stomach and Liver, 

 which are raised to show the Duodenum and Pancreas : 



st, stomach ; p, its pyloric end ; I, liver ; g, gall-bladder ; d r 

 duodenum, extending from the pyloric end of the stomach to 

 the front, where the superior mesenteric artery, sm, crosses the 

 intestines ; pa, pancreas ; sp, spleen ; a, abdominal aorta. 



forming oblong patches about two inches long, 

 are called Peyer's patches. These are affected in 

 typhoid fever. In addition to the follicles of 



