THE INTESTINES. 511 



nerves of the stomach, derived from the vagus and sympathetic, are 

 readily traced into the submucous tissue and they may also be observed 

 entering the muscular layer of the mucous membrane ; but it becomes 

 impossible to follow them further, principally because, in the interior of 

 the mucous membrane itself, they present no more dark-edged tubules, 

 but probably only the pale embryonic fibres.* 



MUCOUS MEMBRANE OF THE SMALL INTESTINE. 



152. The mucous membrane of the small intestine is thinner than 

 that of the stomach, but more complex in its structure, inasmuch as, 

 besides the tubular or Lieberkuhnian glands, it presents a great number 

 of permanent folds and villi ; also, imbedded in its substance, peculiar 

 closed follicles, the so-called solitary and Peyer's glands and, in the sub- 

 mucous tissue of the duodenum, Brunners glands. 



The mucous membrane consists of connective tissue, which is inter- 

 nally homogeneous or indistinctly fibrillated; except where certain 

 glands exist, there is but little submucous tissue, so that it is pretty 

 closely connected with the muscular tunic. Upon the inner surface of 

 the mucous membrane, there rests a cylinder epithelium, to which further 

 reference will be made under the head of the villi; whilst externally, 

 towards the submucous tissue, it is bounded by a layer of smooth muscles, 

 discovered by Brlicke, which measures, at most, 0-0177 of a line ; they 

 are disposed longitudinally and transversely, but in man their slight 

 development renders it often very difficult to discover them. 



153. The villi of the small intestine are small, whitish elevations of 

 the innermost portion of the mucous membrane, readily distinguishable 

 with the naked eye and which, distributed upon and between the valvulce 

 conniventes (Kerkringian valves) through the whole extent of the small 

 intestine, from the pylorus to the sharp edge of the ileo-c0ecal valve 

 (valvula Bauhini), are set so close together as to give the mucous mem- 

 brane its well-known velvety appearance. They are most numerous 

 (50-90 upon a square line) in the duodenum and jejunum, less so in the 

 ileum (40-70 upon a square line). In the duodenum they are broader 

 and less elevated, resembling folds and laminae 1-10-1-4 of a line in 

 height, 1-6-1-2 or even 3-4 of a line in breadth. In the jejunum, they 



* [ In his " Mikroskopische Anatomic," B. II. 2, pp. 149, 153, 164, Professor Koiliker shows 

 that the nvuscular layer of the raucous membrane of the stomach and intestine was discovered 

 by Middeldorpf (De Glandulis Brunnianis, Vratisl. 1846, c. tab.) but remained unnoticed 

 until it was rediscovered by Brilcke and himself. In the small intestine there are, when 

 this muscular stratum is well developed, two layers, though they are not always complete ; 

 the external layer is composed of longitudinal, the internal of transverse fibres. 



In the villi, the smooth muscular fibres have been found not only in many mammalia, but 

 also in birds. The contraction of the villi which they effect appears to have been noticed 

 by Lacauchie, Gruby and Delafond, so long ago as 1842. TRS.] 



