INTESTINES. 



■of a produdion of peritoneum." The two layers of the 

 niefentery feparateat the concave edge of the inteftine, fur- 

 "round it, and are continued into each other at the convex 

 ■fide. It covers, therefore, the whole external furface, except- 

 ing the narrow ftrip where the two layers are fcparated. 



This ferous me:r.brane is very thin, fo as to be nearly 

 tranfparent, and pofTefles no fibres : the mufcular fibres and 

 the blood-vefTeis can be feen through it. Its external furface 

 is moill, and perfeClly fmooth ; while the internal, conneftetl 

 .to the fiibjacent fibres by cellular fubftance, has a rough 



two ftrata of fibres. The external, which is thin and fmall, 

 has a longitudinal dircftion, but their fibres can hardly be 

 fcen except at the convex edge of the gut. The internal is 

 much more confiderable ; its fibres are more numerous, and 

 they furround the inteftine in a circular maimer, fo as to cut 

 the former at right angles. Tliife fibres are the moft dif- 

 tinftly marked about the duodiMuiir. : they grow more flender, 

 poflefs lefs colour, and are therefore hardly obfervable near 

 .the valve of the colon. 



The third or internal covering is a mucous membrane, ge- 

 nerally called by anatomifts the villous coat of the inteftine : 

 it is always covered in a g;-eater or lefs degree with a mucous 

 fiiiid. This is connefted te the misfcular ftratum by a very 

 copious, white, and rather loofe cellular fubftance, the 

 nsrvsus coat of many anatomiils. The trunks of the blood- 

 vefTels and abforbents of the mucous furface are contained 

 in this in great numbers, and give to it the appearance of 

 confiderable vafcularity. It not only connefts the mucous 

 and mufcular coverings together, but alfo unites the dupli- 

 catures of the former. Through the intervals of the muf- 

 cular fibres it joins the cellular fubftance connecting the 

 ferous and mufcular coats. When carefully feparated, it has 

 the appearance of a fine cotton on both furfaces : and this 

 appearance may be ftill more I'.rikingly produced by cutting 

 off the mefentery clofe to the inteftine, inverting the tube 

 and inflating it forcibiy, when the air will penetrate into the 

 ■whole cellular ftrufture of the organ. If the part be ma- 

 cerated, after feparating by dilT-ftion the mucous and muf- 

 cular coats, the water dittends the cells and exhibits the 

 ftrufture very clearly. 



The mucous membrane is much longer than the two other 

 coats, and it is confcquently collefled, throughout the whole 

 canal, into folds, by whiA the difference of length is com- 

 penfated. Thefe are not hke the rugs of the ftomach, 

 formed only in tlie contrafted, and difappearing in the dif- 

 tended ftate of the organ ; but they belong to the efTential 

 ftrufture of the psrt, and exift therefore under all circum- 

 ftances. They are moftly tranfverfe in their direftion ; that 

 is, they cut the long axis of the gut at right angles ; and 

 they are called valvulae conniventes, although they have not, 

 nor can poifibly exert any thing like the aftion of a valve. 

 They begin about an inch from the pylorus, are fmall, and 

 longitudinally difpofed in the duodenum. They then affume 

 a more regular arrangement, are numerous and large, and 

 nearly parallel ; tov-ards the end of the inteftine th^y gra- 

 dually become fmal'er and fewer, and have again in great 

 part of the ileum a longitudinai difpofition. Near the valve 

 of the colon the gut is almoft fmooth. -*» 



The valvulae conniventes may occupy from one to three- 

 fourths of the dianieter of the canal, but rarely extend 

 through the whole circle : they are often joined by fmall 

 communicating folds, and frequently are bifurcated at their 

 extremities. When fliort, they are narrow, and are broader 

 in proportion as they are longer : they are broadeft in the 

 middle and narrower at the extremities. Their breadth may 

 i 



be from half a line to three Knes. They are fo Sompletelf 

 unrefifting, that they yield readily in either direftion to any 

 impelling force, and are applied againft: the furface of the 

 canal. As they depend entirely on the cellular fubftance, 

 they are deftroyed by feparating the mucous membrane and 

 drawing it out : the furface then becomes fmooth, and very 

 much exceeds the length of the other coats. 



The fituation, length andbreadth, direftion, number and 

 form of the valvule conniventes, can be very well obferved 

 in a gut inflated and dried ; but their natural appeai-ance, 

 their thickiiefs, foftnefs, &c. are beft feen in an inverted in- 

 teftine floating in water. 



When the furface of the mucous membrane is attentively 

 examined, it is found univcrfally covered with very delicate 

 and minute folds and projeftions, fo as to give it a flocculcnt 

 appearance ; thefe are named villi. They are more numerous 

 and large in the commencement of tlie cana', and decreafe 

 in number and fize towards the oppofite end. In fome ani- 

 mals, as the dog, thefe villi are remarkably long and nu- 

 merous, fo that the furface of the mucous membrane coai- 

 pietely refembles velvet. 



When the blood-veffels have been fucccfsfully injeft;ed, 

 microfcopical examination difcovers other irregularities on 

 thefe villi, and fliews us that the whole confifts of a clofe 

 net-work of vafcnlar communications. Each villus contains 

 moreover numerous abforbing vefTels, fo that they have been 

 regarded as the great organs of abforption, containing the 

 m.ouths by which the latteals take up the chyle from the 

 inteftine. Anatomifts have bufied themfelves much with 

 attempts to detect the aftuai openings of the lafteals upon 

 the vJli. Lieberkuhn thought that he could fee, by the 

 aid of the microfcope, an opening into each villus leading 

 into a fmall veficular cavity in the centre, which he called 

 the ampulla. Hewfon did not admit this ftrufture, but 

 conceived that the vefTels begin by fimple apertures, in the 

 number of one or more on each villus. Mr. Cruiklhanck 

 met with a fubjeft, in which the lafteals and villi were re- 

 markably loaded with chyle, and he e.\amined thefe parts 

 with the microfcope. Many of tb.e villi were fo full that 

 they feemed like white veficles, without any diftinftion of 

 particular vefTels. «' In fome hundred villi," fays he, *' I faw 

 a trunk of a lafteal, forming or beginning by radiated 

 branches. The orifices of thefe radii were very diftinft on 

 the furface of the villus, as well as the radii themfelves, 

 feen through the external furface, pafCng into the trunk of 

 the lafteal : they were full of a white "fluid. There was 

 but one of thefe trunks in each villus. The orifices on the 

 villi of the jejunum, as doftor Hunter himfelf laid, (when 

 I aflved him, as he viewed them in the microfcope, how 

 many he thought there might be,) were about fifteen or 

 twenty on each villus : and in fome I faw them ftill more 

 numerous." Anatomy of the Abforbing VefTels, chap, ii, 

 and plate 2. 



It has been calculated, that the furface of this mucous 

 membrane, when allowance is made for all its inequaUties, 

 exceeds that of the common integuments. 



Numerous mucous glands are fituated on the external fur- 

 face of the mucous membrane of the fmall inteftine : their 

 dufts open on the internal furface of the organ. Thefe 

 parts are always fmall, and in many cafes are not demon- 

 ftrated without fome difficulty. They are moft advan- 

 tageoufly examined by flitting up the gut, and holding it 

 againft the light ; they are diftinguilhed by their compara- 

 tive opacity. Several exift in the duodenum ; in the jejunum 

 they are fewer, but they incre:d'e in number again in the 

 ileum, are coUcfted into confiderable mafTes, and are par- 

 ticularly numerous towards the end of the inteftine. They 



vary 



