492 



MUCOUS MEMBRANE. 



difficult to reject the belief that it is intimately 

 concerned in the glandular function, and varies 

 in correspondence \vith it. 



To the preceding summary account of these 

 three principal kinds of epithelium much might 

 be added respecting the intermediate forms. 

 Tli is, however, does not appear to be required 

 in so general a description. The spheroidal 

 and the prismatic are seen blended in the speci- 

 men I have figured from the human membrana 

 tympani (fig. 281). 



Of the non-ciliated and ciliated epithelium. 

 The true scaly variety appears never to be 

 clothed with cilia. The prismatic epithelium 

 is that which most commonly bears these vibra- 

 tile organs. They are placed on the free extre- 

 mities of the prisms in the respiratory tract and 

 in the uterus and Fallopian tubes. The true 

 glandular epithelium is always without cilia. 

 This is a general fact, and one of great import- 

 ance. But those varieties which seem interme- 

 diate between the spheroidal and the other two 

 forms are often furnished with cilia ; of which 

 examples maybe seen in the Malpighian bodies 

 of the kidney, in the mucous membrane of the 

 frog's mouth, and in that of the human tympa- 

 num (/g. 281). In all cases the cilia, when 



F':g. 284. 



Various particles of epithelium from the frog's mouth. 



a, b, c, small particles that have not reached the 

 surface. They appear to present three stages or 

 periods, showing a subdivision of the nucleus and 

 a formation of two cells out of one ; rf, three fully 

 developed particles, with cilia on their free sur- 

 face j e,f,g, other complete particles, showing 

 cilia on that part only which has formed a portion 

 of the general surface of the membrane. 

 Magnified 400 diameters. 



they exist, are developed only on that aspect of 

 the particles which forms a portion of the gene- 

 ral surface of the membrane. 



It is as yet entirely unknown by what pro- 

 cess the cilia are produced and nourished ; 

 whether the particles, with their cilia, are shed 

 from time to time, and are succeeded by others, 

 (as is most probable,) or whether the same 

 organs remain, and merely change their com- 

 ponent elements. (On the subject of CILIA in 

 general the reader is referred to Dr. Sharpey's 

 excellent article.) 



Of the elementary tissues appended to the 

 mucous system. The two elementary tissues 

 now described may be considered as the more 



essential constituents of the mucous system, or 

 as forming the simple mucous membrane. This 

 simple mucous membrane envelopes the rest of 

 the body. It contains within its own substance 

 neither vessels nor nerves, but is, strictly speak- 

 ing, extra-vascular. By modifications, chiefly 

 of the epithelial element, it is in itself capable 

 of presenting great variety of appearance and 

 properties in different situations. But in im- 

 mediate connection with its deep surface, that 

 is, with the basement membrane, there are cer- 

 tain tissues common to almost every part of the 

 frame, but here assuming a peculiar arrange- 

 ment and office, and by their diversities in 

 various localities, occasioning the most compli- 

 cated varieties of outward form, of structure, 

 and of function. 



These appended tissues are minute blood- 

 vessels, a lymphatic network, nerves, and areo- 

 lar tissue. 



It has been already stated that in many parts 

 the simple mucous membrane, by its innume- 

 rable minute involutions over an extensive sur- 

 face, is formed into a compound membrane. 

 Into the composition of this (of which a good 

 example is afforded by that of the stomach) the 

 appended tissues enter more or less largely, but 

 they are likewise, in addition, generally spread 

 out in great abundance as a layer underneath 

 the compound membrane. This layer has been 

 commonly termed submucous cellular membrane, 

 (sometimes tunicu nervea,) in the case of in- 

 ternal surfaces, and cutis vtra or dermis in the 

 case of the skin. 



Eloodves-els. These may be said to be 

 universally present under the simple mucous 

 membrane, with the exception perhaps of the 

 cornea, where vessels, in the normal state, have 

 not yet been demonstrated. The capillaries, in 

 their simplest form, appear to be arranged as a 

 plane network, such as that of the rectum of 

 the frog (Jig. 285). The interstices of this 

 network vary much in size and shape in diffe- 

 rent localities. The most copious supply of 

 blood distributed to any such membrane is that 

 afforded to the air-cells of the lungs in all ani- 

 mals. Here this plane capillary plexus has 

 areolae scarcely exceeding the diameter of the 

 vessels themselves. Where the membrane they 

 supply is folded, however irregularly, they 

 follow its surface, and hence result many varie- 

 ties in their arrangement and inosculations. It 

 even seems to be for the purpose of gaining a 

 great freedom of inosculation between the ca- 

 pillaries that the extraordinary complexity has 

 been given to many parts of the simple mem- 

 brane, especially in the secreting glands. For 

 many foldings from somewhat distant parts of 

 the membrane are there brought into imme- 

 diate proximity to one another, and are sup- 

 plied by the same or closely connected vessels. 

 This is remarkably exemplified in the testis, 

 kidney, and liver. The capillary system of all 

 these, as well as of other solid glands, may be 

 styled a solid plexus, being extended in every 

 direction, and presenting areolee of nearly equal 

 size in whatever plane a section of it be made. 

 The liver presents the most perfect instance of 

 such a solid plexus, and in it the vessels are of 



