CHAMBERS'S INFORMATION FOR THE PEOPLE. 



6. DIGESTION IN THE BOWELS. 



After the food, by digestion in the stomach, has 

 been converted into a semi-fluid mass called the 

 chyme (chymos, juice), it passes into the intestine. 

 The length of the human intestine is usually 

 about twenty-five feet. As a general rule it may 

 be stated that the intestines of herbivora are much 

 longer than those of carnivora. This is due to the 

 nature of the food. In those animals, such as the 

 ox, which live on a food very different, physically 

 and chemically, from the tissues of the individual, 

 a complicated digestive apparatus and a long 

 intestinal tube will be found ; whereas those 

 which live on food readily converted into their 

 own tissues (such as a weasel, which preys on 

 the blood of other animals), have a simple 

 digestive apparatus and a short tube. 



The human intestine consists of a convoluted 

 tube, which, from a great change in calibre in two 

 different parts, is divided into (i) the small 

 intestine, and (2) the great intestine. The small 

 intestine is about twenty feet in length, and is 

 divided by anatomists into three portions the duo- 

 denum, jejunum, and ilettm, the latter opening 

 into the great intestine. The whole of this tube is 

 connected with the back of the abdominal cavity by 

 a thin web, called the mesentery, on which blood- 

 vessels, nerves, and absorptive vessels called 

 lacteals, ramify before penetrating and supplying 

 the bowel. When the small intestine is slit open, 

 it presents a large number of transverse folds, 

 which are simply doublings of the mucous mem- 

 brane, so as in little space to increase the surface 

 for absorption. It has also a peculiar velvety 

 appearance, which is due to the fact that it is 

 covered over by innumerable small projections 

 termed villi. They are more numerous in the 

 upper than in the lower portions of the bowel. 

 When examined by the microscope, they are 

 found to be prolongations of the mucous mem- 

 brane, shaped like the finger of a glove, and each is 

 covered by a layer of epithelial cells. In the centre 

 we find the commencement of the true absorbent 

 vessel, called a lacteal, and surrounding it a 

 network of vessels of very minute size. The villi in 

 the small intestine are to a certain extent compar- 

 able to the delicate rootlets of a plant. The latter 

 absorb moisture and soluble nutriment from the 

 soil, while the former are bathed in a nutritious 

 fluid, the chyme, and absorb readily fluids by the 

 blood-vessels, and fatty matters by the lacteals. 

 We find also scattered in large numbers over the 

 mucous membrane, minute tubular glands called 

 Lieberkilhnian glands, after the anatomist Lieber- 

 kiihn, who first described them. In the upper 

 part of the duodenum, there are a few glands, like 

 small clusters of grapes, called Brunner*s glands, 

 the function of which is unknown. 



The great intestine, about five or six feet in 

 length, is so termed because it is so much wider 

 than the smaller one. It is also divided into 

 three parts : the cacum, which is a wide pouch, 

 often of great size in herbivorous animals, and 

 into which the small intestine opens, the entrance 

 being guarded by a valve ; the colon, which forms 

 the greater part of the large intestine; and the 

 rectum, which is situated entirely in the pelvis, 

 and terminates in the anus. The great resembles 

 the small intestine in general respects. 



116 



There is abundant evidence that the function of 

 the villi is connected with absorption, and mainly 

 with the absorption of fatty matters. i. The 

 villi exist only in the small intestine where the 

 absorption of food goes on. 2. They are turgid, 

 enlarged, and opaque during the process of 

 digestion and absorption, and small and shrunken 

 in animals that have been kept fasting for some 

 time before death. 



The function of the muscular coats is to propel 

 the food along the bowel. This they perform by 

 alternate contractions and relaxations, and thus a 

 wave-like motion is produced. This motion may 

 be readily seen in the intestines of an animal 

 recently killed, and is termed a peristaltic 

 action. 



When the food, reduced to a pulpy mass in the 

 stomach, termed chyme, passes into the duodenum, 

 it is mixed with three fluids : I, the bile; 2, the 

 pancreatic juice ; and 3, the intestinal juice. 



The bile is an alkaline fluid secreted by the liver, 

 and, after having been collected in the gall-bladder, 

 finds its way into the upper part of the small intes- 

 tine by a duct, which usually unites with that 

 of the pancreas, and opens by a common orifice. 

 As it flows from the liver, the bile is a thin 

 greenish-yellow fluid, sometimes olive-brown ; but 

 when acted on by the gastric juice, it acquires a 

 distinctly yellow or green hue, hence the appear- 

 ance of vomited bile. Its main use seems to be 

 to promote the digestion of fatty matters, and it 

 accomplishes this end by a peculiar physical action 

 both on the fats and on the intestinal walls, disin- 

 tegrating the former, and impressing on the latter 

 (by moistening the villi) a peculiar condition which 

 facilitates the absorption of fatty matters. The 

 bile separates nutritious matters from those which 

 are non-nutritious, while it stimulates the mus- 

 cular movements of the bowels and arrests putre- 

 faction in the faeces. 



The pancreatic juice is secreted by a long, 

 narrow, flattened gland called the pancreas, or 

 sweetbread, which lies deeply in the cavity of the 

 abdomen, immediately behind the stomach. It 

 is a lobulated or racemose gland, consisting of an 

 immense number of small pouches grouped round 

 the extremities of small ducts. These ducts unite 

 with others, becoming larger and larger, until the 

 great duct of the gland is formed. 



The .secretion is a colourless, clear, somewhat 

 viscid, and ropy fluid, devoid of any special odour, 

 and exhibiting an alkaline reaction. It contains a 

 peculiar principle called pancreatine. 



The function of the pancreatic juice is to emul- 

 sionise the fat of the chyme, and thus promote its 

 absorption. If the duct of the pancreas be tied, 

 and fat be taken as food, a large amount of it will 

 appear in the faeces ; and the same result has been 

 seen in the human being in cases of diseased pan- 

 creas. The pancreatic juice also converts any 

 starchy matter, which may have escaped the 

 action of the saliva, into grape-sugar. 



Of the last of the fluids poured into the intes- 

 tine, the intestinal juice, we know little. It is the 

 aggregate secretion of the various glands which 

 occur in the walls of the smaller intestine. 

 It is a colourless, or sometimes yellowish, ropy, 

 viscid fluid, which is invariably alkaline. It 

 seems to unite in Itself the leading properties of 

 the pancreatic and gastric juices ; that is to say, 

 it resembles the former in converting starch into 



