cxcviii MUCOUS MEMBRANES. 



simply arranged, presents an even surface, as in the tympanum and nasal 

 sinuses, the basement-membrane is absent, at least not demonstrated. In 

 such situations it may possibly have originally existed as a constituent of 

 the corium, and have been obliterated or rendered inconspicuous in conse- 

 quence of subsequent modifications. 



The basement-membrane, as already said, forms the peripheral boundary 

 of the corium ; it is in immediate connection with the epithelium. By its 

 under surface it more or less closely adjoins the fibro-vascular layer. The 

 vessels of the latter advance close up to the basement-membrane, but no- 

 where penetrate it ; the delicate film of which it consists is indeed wholly 

 extravascular. In structure the membrane in question seems perfectly 

 homogeneous, but marks resembling the nuclei of epithelium-cells are some- 

 times seen disposed evenly over its surface, and some observers, considering 

 these as forming an integrant part of the membrane, have looked on them 

 as so many reproductive centres from which new epithelium-particles are 

 generated. Mr. Bowman, on the other hand, considers these objects as 

 nuclei belonging to the undermost and as it were nascent epithelium-cells, 

 which have remained adherent to the really simple basement-membrane. 



The fibro-vascular layer of the corium is composed of vessels both sangui- 

 ferous and lymphatic, with fibres of connective tissue, and, in many parts, 

 of non-striated muscular tissue, variously disposed. The nerves also which 

 belong to the mucous membrane are distributed in this part of its structure. 



The vessels exist universally in mucous membranes, except in that which 

 covers the anterior surface of the cornea ; there the epithelium and base- 

 ment-membrane are present, but, in the adult, no vessels except at the 

 border. Elsewhere the branches of the arteries and veins, dividing in the 

 submucous tissue, send smaller branches into the corium, which at length 

 form a network of capillaries in the fibro-vascular layer. This capillary 

 network lies immediately beneath the epithelium, or the basement-mem- 

 brane when this is present, advancing with that membrane into the villi 

 and papillse to be presently described, and surrounding the tubes and other 

 glandular recesses, into which it is hollowed. The lymphatics also form 

 networks, which communicate with plexuses of larger vessels in the sub- 

 mucous tissue ;~ their arrangement generally, as well as in the villi, has 

 been already noticed. 



The fibres of connective tissue which enter into the formation of the corium 

 are both the white and the elastic. The former are arranged in interlacing 

 bundles, the elastic commonly in networks ; but the amount of both is very 

 different in different parts. In some situations, as in the gullet, windpipe, 

 bladder, and vagina, the connective tissue is abundant, and extends 

 throughout the whole thickness of the fibro-vascular layer, forming a con- 

 tinuous and tolerably compact web, and rendering the mucous membrane of 

 those parts comparatively stout and tough. In the stomach and intestines, 

 on the other hand, where the membrane is more complex, and at the same 

 time weaker in structure, the elastic fibres are wanting and the white con- 

 nective tissue is in small proportion ; its principal bundles follow and sup- 

 port the blood-vessels, deserting, however, their finer and finest branches 

 which lie next the basement membrane ; and accordingly there exists, for 

 some depth below this membrane, a stratum of the corium in which very 

 few if any filaments of the common areolar tissue are seen. In this stratum 

 of the gastro-enteric mucous membrane, the tubular glands with their 

 lining epithelium are set, and between and around them the numerous 

 sanguiferous capillaries and lymphatic vessels are distributed ; but the sub- 



