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DIGESTION 



The Action of the Digestive Juices. By digestion 

 is meant that process by which food is rendered 

 capable of passing through the walls of the digestive 

 system into the blood-vessels and lymphatics, 

 in such a form that it may be of use to, the 

 economy. The most essential change which food 

 undergoes is one of solution. Albumen, starch, 

 fat, and other food -stuffs are quite insoluble in the 

 circulating fluids of the body, and were they in- 

 jected into the blood they would simply block up 

 the smaller blood-vessels. During digestion these 

 pass either into nearly allied chemical substances 

 which are readily dissolved in water, or in the case 

 of fat partly into a soluble soap, and partly into a 

 state of microscopically minute subdivision. It is 

 not sufficient, however, merely to have a soluble 

 food-stuff in order that it may be absorbed and 

 used by the body. Cane (table) sugar is soluble in 

 water, but it is of no use as cane-sugar to the body. 

 If injected into the blood-vessels, it is at once 

 secreted by the kidneys. During digestion it is 

 converted into another sugarless soluble, but in a 

 form which can be used by the economy. 



It must clearly be understood that food intro- 

 duced into the stomach is not received into the 

 system. It is still ' outside the body. ' It is only 

 when it has become absorbed, and has passed in a 

 suitable form, carried by the blood, to the tissues 

 of the brain, muscles, &c. , that it is made use of in 

 so-called vital manifestations. The relief and stay 

 that a meal gives is chiefly due to nervous action, 

 the result of gastric distension. Clay and other 

 non-nutritive matter may be swallowed, and will 

 temporarily produce the same effect. 



The digestion of food is brought about by the 

 action of juices, the saliva, the gastric, pancreatic, 

 intestinal, &c. These are mixed with the finely 

 divided food by the movements of the aliment- 

 ary canal. The digestive juices are in all cases 

 secreted by the microscopic cells which line the 

 various glands opening into the digestive system. 

 The digestive juice, whatever be its source, con- 

 tains either an acid substance or an alkaline one, 

 and in addition a substance termed an unorganised 

 ferment. These ferments differ in many particulars 

 from the organised ferments (bacteria, micrococci), 

 found in putrefaction, diseases such as anthrax, 

 scarlet fever, cholera, &c. The latter are whole 

 living cells, having definite parts or organs. They 

 live and multiply, consume, and excrete in the fluid 

 they inhabit. The digestive ferments are not 

 whole cells, they are the products of cells. They 

 are not the broken-down useless substances, such 

 as carbonic acid, water, &c., which all cells give 

 out, and which an animal, like man ( an aggrega- 

 tion of cells), excretes. They consist of very com- 

 plex active matter, of which we know almost 

 nothing, and which we class with some other sub- 

 stances of which we know little more, under the 

 head ' albuminoids. ' These ferments in fact possess 

 many of the qualities of cells themselves. They 

 are killed by the temperature of boiling water, like 

 all other living things. They have a certain range 

 of temperature, within which alone they evince 

 activity, and within the limits of that range they 

 are more active the higher the temperature. Then 

 again they require the presence of water, in which 

 fluid they are soluble. In their action they seem 

 to be hydrolytic, that is, they produce important 

 changes in the substances they attach by, amongst 

 other things, adding the elements of water. These 

 digestive ferments, together with the other constit- 

 uents of the digestive juices, are secreted by the 

 cells of the digestive glands. If we study these 

 glands and their cells with the microscope, they all 



E resent several points of similarity. Each gland 

 as a duct, which, as in the case say of the salivary 

 glands, divides into several branches. The duct, 



or its branches in the latter case, terminate in the 

 secreting tubules of the gland. These are little 

 tubes, which are lined by the secreting cells, leaving 

 only a small cavity in tne centre of each tubule for 

 the escape of the secretion into the duct. Outside 

 the tubule there is a dense network of fine blood- 

 capillaries, from which the liquor of the blood 

 escapes, bathing the gland and the cells within it. 

 These imbibe the liquor, and they have almost cer- 

 tainly the power of selecting such constituents of 

 it as they require. The substances derived from 

 the blood are used by the cells, which manufacture 

 the juice which they excrete. The constituents of 

 the excretion are not found, it will be observed, 

 in the blood ; they result from chemical processes 

 which go on within the cells, which use blood as a 

 raw material. The cells are very small, being about 

 ^sV^h to yflVffth o f an ^h across. They have 

 each a very thin envelope, within which the finely 

 granular contents of the cell are seen. In the 

 centre is a kernel or nucleus. During digestion 

 the cell enlarges, and granules appear in it. These 

 are the manufactured products : they are discharged 

 into the duct of the gland. 



We have now to consider how it is that the glands 

 are regulated in their secreting powers, how it is 

 that at certain times, and in sufficient quantity, the 

 secreting juice is poured into the digestive system. 

 This is brought about by the action of the nervous 

 system. Although in some cases e.g. the stomach, 

 local nerve-cells placed in the walls of the digestive 

 system itself, seem to regulate in part the secretion, 

 yet in all cases nerve-cells placed in the brain are 

 chiefly concerned. It was thought at one time that 

 the brain acted by causing the Wood-vessels of the 

 gland to enlarge, and in consequence the food- 

 supply and secretion of the gland to become greater. 

 That this is not the whole truth, is shown by the 

 administration of belladonna. If a rather large 

 dose of this drug be taken, the mouth becomes very 

 dry and unpleasant, and it is difficult to speak and 

 to swallow. In other words, the cells of the salivary 

 glands are paralysed, and the customary flow of 

 saliva is for the time being at an end. It is found, 

 however, that during this condition the blood- 

 vessels are by no means contracted. This experi- 

 ment, together with others, suggests that although 

 during the secretion of a digestive juice the blood- 

 vessels dilate, the activity of the secreting cells is 

 not always associated with the condition of the 

 blood-vessels. The latter are undoubtedly under 

 the influence of nerves ( vaso-motor ) which regulate 

 their condition, and it is probable that the secreting 

 cells are themselves under the influence of other 

 nerves, which are special secreting nerves. We 

 have already seen that in the case of the movements 

 of the digestive system, the motor-impulses which 

 started from the central nervous system were called 

 into action by sensory impulses starting from the 

 mucous membrane of the digestive system. So in 

 like manner the nervous impulses which pass to the 

 blood-vessels and the glands are caused, in the first 

 instance, by sensory impulses from the mucous 

 membrane. If food be taken into the mouth, espe- 

 cially if it be ' tasty,' a secretion of saliva rapidly 

 follows, and even the tickling of the mouth by a 

 feather, or the stimulation of the mucous membrane 

 with electricity, will produce the same result. In this 

 case sensory impulses pass to the brain through such 

 nerves as the lingual and glossopharyngeal, and 

 reflexly excite the glands through the medium of 

 the brain, the outgoing impulses passing through 

 nerves such as the chorda tympani (fig. 13). See 

 BRAIN. In the case of salivary secretion, the 

 smell or sight of food may excite a secretion. In 

 this case the sensory impulses pass from the nose 

 or eye along the nerves of these organs to the 

 brain. When the thought of food produces a flow, 



