STOMACH AND INTESTINE. 



347 



large intestine. An" allusion has already been 

 made to the fact that, in many animals, they 

 appear to usurp a portion of the gastric cavity. 

 While the importance which this wide distri- 

 bution would imply, is confirmed by their im- 

 mense number ; which is such that we may 

 estimate their aggregate surface as from ten 

 to fifteen times that of the cylinder of intes- 

 tine into which they open. 



Each tube may be described as a hollow 

 cylinder, having a length which is about five 

 times its width, and ending below in a rounded 



Fig. 256. 



Intestinal tubes from the jejunum, as seen in a vertica 



section. (Magnified 80 diameters.} 

 a, Limitary or basement membrane ; 6, nuclei of 

 the columnar cells which line its interior; c, calibre 

 or cavity of the tube ; d, mouths of the tubes opening 

 into the general cavity of the intestine; e, blind, 

 extremities of the tubes, corresponding to the sub- 

 mucous areolar tissue. 



blind extremity. Its average diameter is 

 about 3-30^ f an i ncn except at its orifice, 

 where it is somewhat wider. The lower part 

 of the tube is often slightly enlarged : and 

 rarely it appears to bifurcate. But while it is 

 doubtful whether these appearances can be 

 depended upon *, it is certain that they are 

 not sufficiently marked or frequent to alter 

 the above general description, -j- 



This cylindrical tube is composed of base- 

 ment membrane and epithelium. The former 

 constituent needs no special description. The 

 latter is a single layer of short columnar cells. 

 It clothes the whole interior surface of the 

 tubes ; and becomes continuous, at their upper 

 extremities, with the epithelium covering the 

 villi, the constituent cells of which generally 

 appear to be somewhat longer in shape. The 

 cylindrical cavity bounded by these cells has 



* See foot-note to p. 321. 



t A more valid exception to the above statement 

 may be found in the upper part of the duodenum 

 of s'ome of the domestic Mammalia ; in whom many 

 of these tubes divide, a little below the surface, into 

 three or four smaller ones. This condition may be 

 regarded as a transition from the structure of either 

 the pyloric tubes, or the clustered glands of Brunn, 

 to that of the ordinary intestinal tube. From the 

 appearances offered by the cylindrical epithelium 

 that lines their interior, the first of these conjec- 

 tures may be regarded as the more probable of the 

 two. 



a diameter which amounts to about one-fourth 

 the width of the entire tube. 



The arrangement of these tubes so precisely 

 recalls that of the gastric glands into which 

 their structure appears sometimes to merge, 

 as scarcely to require any separate descrip- 

 tion. Like these, they are placed vertically 

 side by side, in a sparing quantity of dense 

 fibrous matrix ; and are imbedded by their 

 lower extremities in a layer of a similar ap- 

 pearance. The latter contain much unstriped 

 muscle, the characters of which can be seen 



Fig. 257. 



Vertical and longitudinal section of the small intestine 

 in the lower part of the jejunum, showing the general 

 arrangement of its coats. (Magnified 50 diameters.) 



cr, villi ; b, intestinal tubes ; c, submucous areolar 

 tissue ; d, circular fibres of the muscular coat ; e, 

 longitudinal fibres, external to these, covered by 

 peritoneum. ^ 



even more distinctly than in the analogous 

 gastric structure. The aggregate mass of these 

 vertical tubes forms the great bulk of the 

 mucous membrane. So that a vertical section 

 of this tunic exposes a dense pallisade of 

 tubes, the depth of which corresponds to the 

 thickness of the membrane: while a trans- 

 verse one shows that the interstices of the 

 cylinders are only occupied by a scanty matrix 

 enclosing numerous vessels. 



The chief interruption to their presence is 

 caused by the racemose glands, villi, and fol- 

 licles, which will shortly be described. The 

 ducts of the first of these three structures 

 merely occupy a certain portion of space 

 which would otherwise be taken up by follicles. 

 But the two latter encroach upon the surface 

 of the mucous membrane much more con- 

 siderably. And since it is only between the 

 villi that we find the intestinal tubes, so the 

 number of such tubes which occupy the inter- 

 vals of these processes must necessarily cor- 

 respond to the thickness with which the latter 

 are strewed over the surface. Over the more 

 projecting parts of the follicles, the tubes are 

 also absent ; in a circular space which is sur- 

 rounded by a ring of apertures. The latter 

 belong to the inflected upper extremities of 



