ASSIMILATION. 



225 



1'ie food. The first is the chyle (which means juice 

 or extract), a milky fluid, which is the assimilated 

 mutter ; and the refuse, or that part which is either 

 indigestible or unfit for being converted into chyle. 

 But the latter still contains a portion of chyle., or 

 matter fit for being changed into chyle, and- that is 

 gradually separated in the progress through the re- 

 maining parts of the intestinal canal. 



It appears that vegetable food is acted upon with 

 more difficulty by the gastric juice, in the first part of 

 the process, than animal food ; and it further appears 

 that, after it has been converted into chyme in the 

 stomach, the chyle is with greater difficulty separated 

 from the refuse. For the preparation of vegetable 

 food, by mastication in the mouth, by maceration in 

 preparatory stomachs, or by both, is a much more 

 complicated operation. The intestinal canals of vege- 

 table feeders are also much larger, and the chyme is 

 in consequence subjected to a greater continuance of 

 the action of these viscera. The contents of the in- 

 testines are urged onward, from the entrance to the 

 termination, by a sort of vermicular action, which is 

 called the peristaltic motion, and which bears some 

 resemblance to the motion of the stomach, only it is 

 more progressive. 



The separation of the chyle, which may be 

 considered as the second stage in the process of 

 assimilation, and which continues, though gradually 

 diminishing, through a considerable portion of the 

 intestinal canal, is very obscure in its nature. There 

 are three agencies which may be supposed to be 

 concerned in the production of it : first, the action 

 of the different parts of the chyme upon each other, 

 the efficacy of which is rendered probable by the 

 fact that certain substances are more nutritious when 

 taken into the stomach together than either of them 

 is when taken singly ; secondly, the influence of the 

 bile and pancreatic juice ; and thirdly, the action of 

 a peculiar secretion of the inner coat of the intestine 

 itself. Whether one, or another, or all of these pro- 

 duce the effect, or if all, how much is to be attributed 

 to one, and how much to another, is not known. Thus, 

 in this part of the process of assimilation also, we are 

 left in doubt both as to the agent and the specific effect. 



From the intestinal canal, and especially from the 

 duodenum, in which it is most copiously produced, 

 the chyle is taken up by the lacteal vessels. These 

 are small tubes which open on the inner coat of the 

 intestine, not by mouths equal even to their small 

 diameters, but each by a number of very minute 

 pores or villi, which radiate from a centre, and whose 

 openings are so small that they admit only the most 

 minute substances. These lacteals with their villous 

 openings continue along the greater part of the intes- 

 tinal canal, but they become less numerous, as the 

 food advances, and furnishes a smaller quantily of 

 chyle. From the intestine they proceed along the 

 mesentery, uniting into thicker trunks, and also anasto- 

 mosing with each other, so as, in some instances, to 

 form a sort of network. They also contain numerous 

 valves, and as the seats of these do not expand along 

 with the intermediate parts, the distended lymphatics 

 appear to consist of a succession of little barrels or 

 beads. 



All the lacteals discharge their contents into the 

 lower extremity of the thoracic duct, which ascends in 

 the back part of the thorax, and pours its contents 

 into the left subclavian vein near its junction with 

 the heart. Thus the assimilated product of the food 



NAT. HIST. VOL. I. 



is mixed with the mass of the blood. In their course 

 from the intestine to the thoracic duct, the lacteals 

 pass through one or more glands ; but whether their 

 contents undergo any change, and if any, what that 

 change is, are obscure points. The chyle differs from 

 blood in many respects, one of the most obvious of 

 which is a considerable admixture of sugar and water. 



The chyle does not go to the blood alone, but 

 mingled with the contents of the lymphatic vessels, 

 which appear to fetch their colourless contents from 

 all parts of the body, and united enter the thoracic 

 duct ; but as the fluid which they contain comes from 

 the living parts of the animal, it must be considered 

 as a previously assimilated substance ; but whether it 

 again enters into the circulation, and if so, what office 

 it performs, are points upon which we have no infor- 

 mation, and they form no part of the process of assi- 

 milating. 



The blood with which the chyle, or new matter, is 

 mixed, does not immediately go over the body in the 

 course of the systematic circulation. It passes im- 

 mediately into the right auricle of the heart, thence 

 it passes into the right ventricle, where the valves be- 

 tween the two prevent its return. The contraction 

 of the ventricle sends it along the pulmonary artery 

 to the lungs ; and after undergoing the action of the 

 air inspired in breathing there, it returns by the pul- 

 monary vein to the left auricle, thence to the left 

 ventricle, and by the action of that and the systematic 

 arteries, it is sent all over the body. The quantity 

 of chyle which mixes with the blood at each pulsation 

 of the heart must be very small, as there is no trace 

 of its colour in the blood which is sent to the lungs, 

 which has the dark colour of venous blood. But the 

 minute division of the blood in its passage through 

 the lungs must tend to the intimate union of the 

 chyle with it, and it may be possible also that the 

 action of the lungs is necessary to perfect the assimi- 

 lation, and finally convert the chyle into blood, but 

 the agent and the process in this final step of the pro- 

 cess are just as obscure as in the two preceding ones. 



We have now followed the progress of the food 

 from its first entrance by the mouth of the animal to 

 its union with the mass of the blood, which is under- 

 stood to be the fluid which supports the growth and 

 repairs the waste of all the parts of the body. In 

 this we have done nothing more than give a simple 

 outline of the process of digestion ; but this is, in 

 truth, all that can be given. We have seen that, 

 besides the merely mechanical preparation of the 

 food, there are three distinct operations : the forma- 

 tion of chyme in the stomach, the separation of chyle 

 in the intestines, and the turning of that chyle into 

 blood after it has passed into the subclavian vein. 

 There may be others, by the glands through which 

 the lacteals pass, and the union of the lymph with 

 the chyle in the thoracic duct ; but instead of know- 

 ing how these take place, we have no evidence of the 

 fact that they take place at all ; and therefore it would 

 be unwise to darken further with them a subject 

 which in its own nature is abundantly obscure. Our 

 inquiry reduces the assimilation to three distinct pro- 

 cesses or acts, and we can tell plainly enough what 

 takes place at each of them ; but in none of them 

 can we tell how, or by what agency. We can indeed 

 bring all three home to the living animal, by show- 

 ing that they are neither chemical nor mechanical , 

 from which it follows that they could not be performed 

 by the properties of mere matter, or originate in any 



