504 SPECIAL HISTOLOGY. 



form a continuous layer, since they are very few or entirely absent along 

 the attachment of the mesentery ; they are usually most distinct upon 

 the free border, though even here they may be readily torn away with 

 the serous membrane, so as, at once, to leave the second layer exposed. 

 The latter is complete and continuous, consisting of circular bundles, 

 which not uncommonly anastomose at very acute angles. 



In the large intestine, the longitudinal fibres are reduced to the three 

 ligamenta coli, muscular bands of 4-6, or even 8 lines broad, and -J-J 

 line thick, which commencing upon the caecum are united upon the 

 sigmoid flexure, into a single longitudinally fibrous layer, which is con- 

 tinued upon the rectum. Beneath these bands there lies a continuous, 

 circularly fibrous layer, thinner than in the small intestines and more 

 especially developed in the duplicatures, which are known under the 

 name of the plicae sigmoidece. 



Fig. 201. The rectum possesses a muscular layer of 1 line and more 

 thick, in which the more abundant longitudinal fibres lie ex- 

 ternal to the circular. The ultimate, somewhat thickened ex- 

 tremity of the circular fibres is the sphincter ani internus, with 

 which the transversely striated sphincter externus and levator 

 ani are conjoined. 



In their elementary structure, all the muscles of the proper 

 alimentary canal belong to the so-called smooth or non-striated 

 (vegetative, organic) muscles (see 26). Their elements, the 

 fibre-cells, are fusiform, on the average 0-002-0*003 of a line 

 broad, and O'06-O'l of a line long, pale, flattened, and homo- 

 geneous, and provided with a nucleus 0-006-0-012 of a line 

 long, and 0-001-0-0028 of a line broad. 



Many of the fibres present knot-like enlargements and fre- 

 quently zigzag flexures, which produce the transversely striated 

 appearance of the entire bundles of such muscles so frequently 

 met with in spirit preparations. The arrangement of the 

 fibre-cells in the different strata is simply this ; mutually applied 

 in their length and breadth and coherent, they are united into 

 thin muscular bands, which then, invested with a coating of 

 connective tissue and, frequently, also united into secondary 

 bundles, constitute the thinner or thicker muscular tunics of the 

 different regions ; which, again, are surrounded and separated 

 from the contiguous parts, by considerable layers of connective 

 tissue. 



Bloodvessels are very abundant in the smooth muscles ; and 

 their capillaries, of 0-003-0-004 of a line, constitute a charac- 

 teristic* network with rectangular meshes. 



FIG. 201. Muscular fibre cell from the small intestine (human). 



* [Hardly characteristic ; the vessels are arranged in precisely the same way in the fascial 

 aponeuroses; e. g., the/ascia lata of the thigh. TRS.] 



