402 STRUCTURE OF ALIMENTARY CANAL. [BOOK n. 



the whole of the epithelium is extra-vascular. The connective 

 tissue where it touches the cells forms a more or less continuous 

 sheet; this is often spoken of as the basement membrane and 

 may be regarded as the demarcation between the extra-vascular 

 epithelium and the vascular connective tissue basis. The two 

 together, the epithelium and the connective tissue basis, form what 

 is known as the mucous membrane. 



At the bases of the cylindrical cells, wedged in between them 

 and the basement membrane, may be seen, in certain situations 

 distinctly, in other situations less distinctly, small cells, that is to 

 say cells the body of which is small relatively to the nucleus. 

 These have been supposed to be young cells, held in reserve to 

 replace any of the larger cylindrical cells which may from time to 

 time disappear; but it has been maintained on the other hand 

 that, in some situations at least, the cylindrical cells are renewed by 

 division, and that these small cells are in reality leucocytes which 

 have " wandered " into the epithelium. 



Outside the mucous membrane or mucous coat is placed the 

 thick muscular coat. This consists of two layers of plain muscular 

 fibres, an inner thicker layer, in which the fibres and bundles of 

 fibres are disposed circularly round the lumen of the alimentary 

 canal, and an outer thinner one, in which the fibres are disposed 

 longitudinally. The bundles and sheets of fibres (see 89) are 

 bound together by connective tissue carrying blood vessels, lym- 

 phatics and nerves, and a thin sheet of connective tissue more or 

 less distinctly separates the thicker inner circular muscular coat 

 from the thinner outer longitudinal muscular coat. 



The lower or outer part of the mucous membrane where it 

 becomes attached to the muscular coat is formed of very loose 

 connective tissue, the interspaces of the bundles being large and 

 open. This is spoken of as the submucous tissue or submucous 

 coat. It is so loose that the mucous coat can easily move over the 

 muscular coat, and along it the one can easily be torn away from 

 the other, more easily in some parts of the canal than in others. 

 It carries the larger arteries and veins, whose smaller branches 

 and capillaries pass into and from the mucous membrane. Lying 

 in the mucous membrane at some little distance from the epithelium 

 is found a thin layer of plain muscular fibres, called the tunica 

 muscularis mucosce. It is more conspicuous in some situations 

 than in others, and when complete consists of an inner single layer 

 of fibres disposed circularly and an outer single layer of fibres 

 disposed longitudinally. The connective tissue on the inside of 

 the muscularis mucosse, between it and the epithelium, is generally 

 of a somewhat different character from that outside the muscularis 

 mucosae, and in many places is of the kind called adenoid or 

 reticular tissue ; of this we shall hereafter have to speak. 



Lastly, from the stomach to the rectum the muscular coat of 

 the alimentary canal is covered by the visceral layer of the peri- 



