370 



THE POPULAR EDUCATOR. 



dition especially has a very unfavourable influence upon the 

 proper performance of this function. 



The effect produced upon the food by the gastric juice is 

 confined almost entirely to the nitrogenous elements ; these, 

 whether derived from the animal or vegetable kingdoms, *are 

 all reduced to a low form of albumen, which has the power of 

 transuding, or passing through, an animal membrane, and thus 

 being absorbed a power in which ordinary albumen is deficient. 

 The gastric juice would appear not to have any action on the 

 starchy elements, though the change of these into sugar con- 

 tinues after they have entered the stomach, probably owing to 

 the saliva that is mixed and swallowed with them. Nor are 

 the fatty constituents altered, save in being reduced to a finer 

 state of division by the cell-walls being dissolved and the fat- 

 globules set free. 



It has been already stated that the walls of the 'stomach 

 reduce the food into a semi-fluid state before it leaves that 

 organ ; at this stage it is known as the chyme, which is then 

 propelled against the pyloric opening, which relaxing, allows it 

 bo escape into the intestine. It will be remembered that it 

 ivas into the duodenum, the commencement of the small intes- 

 tine, that the common duct of the liver and pancreas poured 

 their secretions, and it is here therefore that the food, or rather 

 chyme, comes under the influence of these fluids. The pan- 

 creatic juice is in composition almost identical with saliva. 

 When pure, it is transparent, colourless, slightly viscid, and 

 contains an active principle called pancreatine. Like saliva, 

 it has the power of converting the starchy principles into 

 glucose, or grape sugar, only that it is a much more powerful 

 agent for that purpose ; but, in addition to this, the pancreatic 

 juice acts strongly upon the fatty elements, breaking them up 

 into extremely minute particles, so that when mixed with the 

 other fluids they form a kind of emulsion, and in this form are 

 easily absorbed. These are the main actions of the pancreatic 

 juice ; but there seems to be a growing opinion that they are 

 not the only ones, and that it is a much more influential 

 agent in promoting digestion than has been hitherto held. 



The next change produced in the chyme is that caused by 

 the presence of bile the secretion of the liver. The bile 

 is a very complex liquid, as to the composition and uses of 

 which there have been more investigation and discussion than 

 concerning any other secretion of the body, and with less de- 

 finite results. It is a somewhat viscid fluid of a greenish- 

 yellow colour, strongly bitter taste, and a peculiar nauseous 

 smell. Its specific gravity is from 1026 to 1030. When 

 secreted it is neutral, but after decomposition becomes eventually 

 alkaline. It contains about 14 per cent, of solid matter, the bulk 

 of which is composed of compounds of what are known as biliary 

 acids, with a soda base ; it also contains a considerable por- 

 tion of a crystalline fat, cholesterine, which forms the bulk of 

 the concretions called gall-stones. The process of secreting 

 bile, unlike the gastric juice, etc., is continuous, though it is 

 probably somewhat accelerated on taking food. The quantity 

 secreted during the twenty-four hours is variously estimated, 

 some authorities placing it as high as three or four pounds ; 

 ethers consider six to eight ounces the probable amount. The 

 part it performs in the function of digestion is very uncertain, 

 and, from the most recent investigations, is thought to be of 

 much less importance than was formerly believed. It is said 

 that it has some influence in emulsifying fat, and thus render- 

 ing it capable of being absorbed ; and also that, by its alka- 

 linity, it helps to neutralise the acidity the chyme derives 

 from the gastric juice ; and it may, perhaps, have some in- 

 fluence on the starchy elements of foods. It is undoubtedly 

 of much greater importance as an excretion of the body, 

 serving to carry out of the system some of the effete 

 material. Its absolute necessity in this capacity is made 

 manifest by the great derangement of health that is pro- 

 duced even by a temporary obstruction to its secretion. Like 

 the gastric juice, bile is strongly antiseptic, and thus helps to 

 prevent the decomposition of food during its sojourn in the 

 intestines ; it has also been considered, by promoting the secre- 

 tion of the intestinal glands, to act as a natural purgative. 

 This is pretty well all that can be surely said as to the purpose 

 of the bile, and it must be confessed that our knowledge con- 

 cerning it is still in a very imperfect and unsatisfactory state. 



As the food passes down the intestines, mixed with the bile 

 and pancreatic juice thenceforward taking the name of the 



chyle it comes in contact with the secretions of the numerous 

 smaller glands that thickly stud this part of the alimentary 

 canal. This intestinal mucus, especially that of the small in- 

 testines, seems to combine the properties of the gastric and 

 pancreatic fluids, and acts powerfully on the starch, fat, and 

 albuminous substances. 



We have now traced the food into the large intestine, and 

 must try back and see how the digested part of it is con- 

 veyed into the general system, and it will be needful first to 

 describe the organs specially provided to carry these nutritive 

 elements into the circulation. When describing the mucous 

 membrane of the small intestines, attention was called to the 

 multitude of little projecting points with which it was covered, 

 and it was said that each of these processes, or villi, consisted 

 of a fold of mucous membrane enclosing a loop of blood-vessels, 

 and of another series of vessels called lacteals. These latter are 

 the ones we have now to consider. They derive their name from 

 the milky appearance of the fluid which they contain. In struc- 

 ture they are exceedingly delicate, having walls so transparent 

 that the fluid they convey is readily seen through them. The 

 smaller branches run together, and join to form two or three large 

 trunks, which eventually empty themselves into a common reser- 

 voir the thoracic duct. Vessels of a very similar kind, called 

 lymphatics, are scattered almost universally throughout the 

 body, and also empty themselves into the same reservoir. They 

 all in their course, lacteals as well as lymphatics, pass through 

 a number of glandular structures, by which their contents are 

 elaborated and in some measure prepared for their ultimate 

 purpose. 



The thoracic duct, which conveys the great mass of the lymph 

 and chyle into the blood, is about the size of a goose quill, and 

 about eighteen or twenty inches in length. It lies deeply in the 

 abdomen at its commencement, starting from the front of the 

 second lumbar vertebra, and passing up along the front of the 

 spine to the root of the neck, empties itself into one of the large 

 veins a little distance from where it joins the heart. 



The process of absorption commences in the stomach. Here 

 those elements which are liquid, or are perfectly dissolved, as 

 the saline or saccharine, pass at once through the walls of the 

 blood-vessels of the mucous membrane of the stomach, and thus 

 enter the circulation ; and not only these, but the starchy ele- 

 ments, having been changed by the saliva, and the albuminous 

 by the gastric juice, are also partially directly absorbed ; but 

 though absorption takes place considerably in the stomach, it is 

 immensely increased when the chyme reaches the small intestine ; 

 for here, in addition to the blood-vessels, which, as in the sto- 

 mach, take up such of the saccharine and albuminous elements 

 as have escaped from that organ, it comes in contact with the- 

 lacteals or absorbents proper. These, though they do absorb 

 the albuminous and other elements, appear specially to act upon 

 the fatty constituents. As the food passes on through the ali- 

 mentary canal, more and more of it becomes absorbed, so that 

 by the time it reaches the lower part of the large intestine, 

 only about one-sixth, made up of insoluble and innutritions 

 matter, together with the waste products of the body remains- 

 to be rejected by the system. 



The blood, which receives from the stomach and intestines 

 the products of digestion, does not pass directly into the general 

 circulation, but, as has been already indicated, is conveyed by 

 the portal vein to the liver, there to be acted upon by that 

 gland, and have eliminated from it certain principles which 

 would be noxious to the system, and also probably to have some 

 of its elements so changed that they shall more nearly approxi- 

 mate to the composition of the blood. The chyle also which 

 the lacteals convey undergoes several changes by its passage 

 through a series of gland-structures. If examined soon after its 

 absorption, it is found to contain albumen having no power of 

 spontaneous coagulation ; and it also holds in suspension a large 

 quantity of fatty matter in an extremely fine state of division. If 

 it be examined at a later date, after it has passed through some 

 of the glands which have been mentioned, it will be found that the 

 albumen has diminished, and that in its place is another sub- 

 stance called fibrine. The presence of this, when the chyle is ex- 

 posed to the air, causes it soon to coagulate in a semi-solid mass. 

 The oil-globules will also be found to have diminished, and a 

 number of peculiar floating cells, or chyle-corpuscles, have made 

 their appearance. The fluid called lymph, which the lymphatics 

 absorb from all parts of the body, and pour into the thoracic 



