Ill 



quently capable of greater extension. These longitudinal fibres being* 

 besides, shorter than the intestine, crease it transversely, and form with- 

 in it a number of cavities and cells, marked outwardly by prominences, 

 separated by depressions. If, in addition to the peculiarities of struc- 

 ture, it be considered, that in the ccecum and a great part of the colon, 

 the contents of the bowels have to ascend against their own weight; that 

 the curvature forming the sigmoid flexure of the colon is very considera- 

 ble, and that, in short, (he rectum before its outer termination in a nar- 

 row aperture, is considerably dilated, it will be evident, that in the great 

 intestines, every thing tends to protract the stay of the excrements. 



The appenclicula vermi-formis of the coecurn is, in man, too small to 

 perform this office; in the herbivorous quadrupeds, in which it is much 

 larger, and sometimes not single, it may serve as a reservoir to the faecal 

 matter. Its existence merely shows, in man, an analogy to those animals 

 in which it is truly useful, and it concurs in manifesting, that Nature, in 

 the formation of particular organs, in certain kinds of animals, aims at a 

 mere outline which she fills up in others, to show, as it were, that there 

 are points of resemblance between all beings whom she has gifted with 

 life and motion*. 



While in the great intestines, the alimentary substance becomes mere- 

 ly faecal, by parting with the small quantity of chyle which it may yet 

 contain. The number of the absorbents decreases progressively from 

 the coecum to the rectum ; the small number of these vessels, accounts 

 for the difficulty of throwing in nourishment by means of clysters, when 

 there is an obstruction to deglutitionf. The excrements thicken, harden, 

 and become formed or moulded, in the cells of the colon, they are then 

 urged by the peristaltic action, into the rectum, in the cavity of which 

 they accumulate, till they excite on its parietes an action which deter- 

 mines their expulsion. 



XXX. Of the evacuation of the faces. When a call to evacuate the faeces 

 is experienced, the rectum contracts, while the diaphragm descending, 

 .and the abdominal muscles receding towards the spinej, thrust the vis- 

 cera of the abdomen towards the cavity of the pelvis, and compress the 

 intestines which are filled with fsecal matter. During these efforts, the 

 perineum perceptibly descends, and the fibres of the levator ani are 

 somewhat elongated. The combined action of the rectum and of the 

 abdominal muscles, overcomes the resistance of the sphincters, and the 



* The number of these appendages is very great in some of the interior animals, 

 especially in some of the fishes. See Cuvier, An. Comp. Godman- 



\ Some physiologists have considered as unnecessary, this concurrent action of" the 

 diaphragm and abdominal muscles ; they ground their opinion on the circumstance, 

 that animals whose abdomen has been laid open are capable of voiding their faeces. 

 Astruc, one of the luminaries of Montpellier, denies the action of the abdominal mus- 

 cles, in the efforts which one makes at stool, and in support of his opinion, he brings 

 forward this geometrical proposition, " that a cord disposed in the form of a circle, 

 can, by contracting, shorten itself in an infinitely small degree, and, therefore, not 

 perceptibly." On which Pitcairn humorously enough observes, that Astruc had never 

 practised what he reasons upon : ." credo Jlstrucciwn nunquam cacasse. Jluthor's 

 Note. 



* Tliis is one, but not the only reason, why the system cannot be sustained for any 

 length of time by injections. For the formation of chyle, containing the elements of 

 blood, which it always does when genuine, it is indispensably necessary that the aliment 

 should be previously converted into chyme, an operation -which seems, from experi- 

 ments, cannot be effected in the great intestines. Chapman. 



