ABSORPTION. 



29 



that, except under peculiar circumstances, they 

 reject every other substance.* 



When the elements of the chyle have been 

 received into the lacteals, it appears to undergo 

 a certain degree of elaboration, by which it is 

 farther assimilated and perfected, an operation, 

 the intimate nature of which we are unable to 

 explain, but which, as well as its entrance into 

 the mouths of the vessels, we correctly refer to 

 their vital action. After the chyle has entered 

 the lacteals, there is less difficulty in conceiving 

 the subsequent steps of the process. We are at 

 least able to generalize the operation, by referring 

 it to contractility, the same power which ori- 

 ginates motion in other parts of the system. 

 It must, no doubt, be admitted, that the exis- 

 tence of the muscular fibres of the lacteals has 

 not been satisfactorily demonstrated, and that, 

 until this has been accomplished, our opinion 

 can only be regarded as hypothetical : but we 

 have here the advantage of being able to assign 

 a probable and sufficient cause of the effect, 

 and are aware of the point towards which we 

 must direct our future investigations^ Before 

 we conclude this branch of the subject, we may 

 remark concerning the contents of the lacteals, 

 that, under ordinary circumstances, we have 

 no decided proof of these vessels containing 

 any substance except the elements of the chyle, 

 and that, although in some of the experiments 

 referred to above, extraneous bodies have been 

 occasionally found in them in minute quantity, 

 these cases must be regarded as exceptions to 

 the general fact. 



With respect to the chyle itself, it has been 

 a subject of examination by the chemists, whe- 

 ther its properties are always uniform in the 

 same animal, or class of animals, under the 

 various circumstances of age, constitution, and 

 still more of diet, to which they are subject. 

 But it may be necessary, before we enter upon 

 this inquiry, to premise a few remarks upon 

 the meaning of the terms chyme and chyle. 

 By the older physiologists they were very gene- 

 rally employed as synonymous, and this is still 

 the case with some of the modern writers, more 

 especially on the continent.^ A clear distinc- 

 tion between them has, however, been pointed 

 out and recognized, and as there appears to be 

 an essential difference between them, it is desi- 

 rable that it should be generally adopted. The 

 first of these substances is the immediate pro- 

 duct of the action of the gastric juice on the 

 aliment, as received into the proper digestive 

 stomach, while the latter is the substance which 

 is produced by a subsequent part of the pro- 

 cess of digestion. The conversion of chyme 



* See the remarks of MM. Chaussier and Ade- 

 lon, ubi supra, p. 272 et seq. ; also Adelon, 

 Physiol. t. iii. p. 85 et seq. ; and Alison's Out- 

 lines, p. 79. 



t This is essentially the doctrine of Haller, Prim. 



. Lin. c. xxv. .568. Sheldon, p. 28, and Cruik- 



shank, c. 12, are advocates for this doctrine ; but it 



is opposed by the high authority of Mascagni, ps. i. 



sect. 4. p. 27, 8. 



t This appears to be the ca^e with M. Rullier, 

 art. " Chyme/' in Diet, de Med. t. v. p. 241 . . 4 ; 

 M. Addon, however, clearly marks the distinction, 

 Physiol. t. iii. p. 25, et alibi. 



into chyle seems to commence shortly after it 

 leaves the stomach, and while it still remains 

 in the duodenum, is so far advanced as to be 

 reduced into a condition proper for being re- 

 ceived into the lacteals. There is, however, 

 reason to believe that the completion of the 

 process takes place in the lacteals themselves, 

 and even that it is not until the chyle arrives at 

 the thoracic duct, or at least at the great trunks 

 of the lacteals, that it is fully elaborated. The 

 nature of the change which the chyme expe- 

 riences in the duodenum, and the agents by 

 which this change is effected, what share the 

 secretions of the part itself, the bile, or the 

 pancreatic juice have in the operation, are 

 questions that still remain in discussion, and 

 which will be considered in the appropriate 

 parts of this work.* 



For the analysis of the chyle we are prin- 

 cipally indebted to Vauquelin, Marcet, and to 

 Dr. Prout. Vauquelin employed the chyle 

 of the horse, as taken from the large trunks 

 of the lacteals and from the thoracic duct.f 

 The experiments of Marcet were principally 

 directed to the inquiry, how far the chyle of 

 the same kind of animal was affected by dif- 

 ferences in the diet, according as it consisted 

 Eincipally of animal or vegetable substances.l 

 r. Prout's experiments on the chyle extended 

 both to its general properties, and to the dif- 

 ferences produced by different kinds of diet, 

 while, in addition to these points, he entered 

 into a very interesting examination of the suc- 

 cessive changes which it experiences, from its 

 first entrance into the lacteals until its final 

 deposition in the thoracic duct. The result of 

 these experiments, as far as our present inquiry 

 is concerned, tends to shew that the vegetable 

 chyle differs somewhat, in its physical and 

 chemical properties, from that of animal origin, 

 and that the chyle, when it first enters the 

 lacteals, is in a less perfect state, while it be- 

 comes more assimilated to the blood in pro- 

 portion as it advances towards the thoracic 

 duct. 



With respect to the means by which the 

 animalization of the chyle is perfected after it 

 enters the vessels, we have no certain informa- 

 tion, and we have scarcely any analogy which 

 may assist in guiding our opinion. What is 

 termed by modern physiologists the action of 

 the vessels, by which so many operations of the 

 animal economy are supposed to be effected, 

 we may regard rather as an expression which 

 serves as a convenient veil for our ignorance, 

 than as throwing any light upon the process. 

 We have no evidence that any addition is made 

 to the chyle while in the lacteals ; and indeed 

 we can scarcely suppose it possible that this is 

 the case, so that the only conceivable effect of 

 this action is reduced to the motion which is 

 imparted to the chyle by the alternate contrac- 

 tion and relaxation of the vessels, in conse- 



* See the remarks of Adelon, art. " Chyliferes." 

 Diet, de Med. t. v. 



t Ann. China, t. Ixxxi. p. 113 et seq. ; Ann. Phil. 

 v. ii. p. 220 et seq. 



J Med. Chir. Trans, v. vi. p. 618 et seq. 



Ann. Phil. v. xiii. p. 2^ . . 5. 



