OF ANIMAL BODIES. 101 



the chyle, now flowing along with the old blood, to the 

 heart. This whole route can be exhibited to the eye. 

 Nothing is left to be supplied by imagination or conjec- 

 ture. Now, beside the subserviency of this whole struc- 

 ture to a manifest and necessary purpose, we may remark, 

 two or three separate particulars in it, which show, not on- 

 ly the contrivance, but the perfection of it. We may remark, 

 first, the length of the intestines, which, in the human 

 subject, is six times that of the body. Simply for a pas- 

 sage, these voluminous bowels, this prolixity of gut,"^ seems 

 in no wise necessary ; but, in order to allow time and space 

 for the successive extraction of the chyle from the digested 

 aliment, namely, that the chyle, which escapes the lacteals 

 of one part of the guts, may be taken up by those of some 

 other part, the length of the canal is of evident use and 

 conduciveness. Secondly , We must also remark their peristal- 

 tic motion ; which is made up of contractions, following 

 one another like waves upon the surface of a fluid, and not 

 unlike what we observe in the body of an earthworm crawl- 

 ing along the ground ; and which is effected by the joint 

 action of longitudinal and of spiral, or rather perhaps of a 

 great number of separate semi-circular fibres. This cu- 

 rious action pushes forward the grosser part of the aliment, 

 at the same time that the more subtile parts, which we call 

 chyle, are, by a series of gentle compression, squeezed in- 

 to the narrow orifices of the lacteal veins. Thirdly, It 

 was necessary that these tubes, which we denominate lac- 

 teals, or their mouths* at least, should be made as narrow 

 as possible, in order to deny admission into the blood to 

 any particle, which is of size enough to make a lodgement 

 afterwards in the small arteries, and thereby to obstruct 

 the circulation : and it was also necessary that this extreme 

 tenuity should be compensated by multitude ; for a large 

 quantity of chyle (in ordinary constitutions, not less, it has 

 been computed, than two or three quarts in a day) is, by 

 some means or other, to be passed through them. Accord- 

 ingly, we find the number of the lacteals exceeding all 

 powers of computation ; and their pipes so fine and slen- 

 der, as not to be visible, unless filled, to the naked eye ; 

 and their orifices, which open into the intestines, so small, 

 as not to be discernible even by the best microscope. 

 Fourthly, The main pipe which carries the chyle from the 

 reservoir to the blood, viz. the thoracic duct, being fixed in 

 an almost upright position, and wanting that advantage of 

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