490 



MUCOUS MEMBRANE. 



Fig. '278. 



Vertical section of the epithelium of the mouth , shelv- 

 ing its lamellae and the changes of form which the 

 particles successively undergo. 



a, superficial laminae, consisting of true scales ; 

 b, c, particles in progress of flattening-; d, deep 

 layer of particles ; a', b', c', d', separate particles 

 in the several stages. Magnified 300 diameters. 



Fig. 279. 



A few scales detached from the surface of the uvula. 

 Magnified 300 diameters. 



the homology of the mucous membranes with 

 the skin. The term ' scales ' is only applicable 

 to these particles in the last of the stages 

 through which they pass. They first appear on 

 the surface of the basement membrane as gra- 

 nular dots, each of which soon becomes in- 

 vested with a cell membrane. Both nucleus 

 and cell increase in size up to a certain point, the 

 cell being then more or less globular, and con- 

 taining a material that appears transparent and 

 almost entirely fluid. By this circumstance, 

 chiefly, it is distinguished from the spheroidal 

 form of particle, presently to be noticed. The 

 cell now begins to flatten, loses its fluid con- 

 tents, and is at the same time the seat of certain 

 changes by which its chemical properties are 

 modified. At length its opposite surfaces unite, 

 except where the nucleus intervenes, and a 

 lamella of extreme tenuity results, which being 

 now arrived at the surface is loosened and 

 shed. It appears to be by the continual pres- 

 sure arising from the growth of newly-formed 

 particles that the peculiar characters of this 

 variety result. Accordingly, the scales are 

 only found constituting the superficial layers of 

 a series (Jig. 278, aa '). It is met with in those 

 parts only where foreign pressure, or more pro- 

 perly friction, has to be encountered. In such 

 parts i\ thick coating of epithelium is evidently 

 desirable, and the hard and almost horny 

 qualities which these particles at length assume 

 where most exposed to violence, admirably 

 adapt them for their object. On such parts, 

 moreover, cilia would not be needed, and it 

 would even seem that this variety of epithelium 

 when converted into true scales possesses 



neither sufficient substance nor vital power to 

 develope and support these exquisite organs. 



The scaly epithelium is remarkable for the 

 tenacity with which its particles adhere to one 

 another, and to the surface on which they rest. 

 This adhesion is manifest at all the stages 

 through which the particles pass. It is stronger 

 between particles at the same stage than be- 

 tween those at different stages of growth, so 



. O D ' 



that there is always a tendency to a separation 

 into successive laminae on maceration or other- 

 wise. Hence have resulted the divisions of 

 the epidermis into two, three, or more layers, 

 and especially that remarkable fallacy of re- 

 garding the rete mucosum as a distinct structure. 

 How far this adhesion is owing to the presence 

 of an intercellular substance in all instances 

 it is difficult to decide ; but it seems highly 

 probable that, in the deepest layers, where the 

 particles are small and rounded, such asubstance 

 must exist in considerable abundance, filling up 

 the interstices, and serving as a kind of blastema, 

 in which the nuclei (or cytoblasts) of fresh parti- 

 cles originate. I have lately (Jan. 1842) ascer- 

 tained a very curious fact, giving evidence of this 

 adhesion. This is, that the delicate threads 

 drawn out of the cutis \\hen the cuticle is 

 stripped from a piece of macerated skin, con- 

 sist entirely of the epithelium of the sweat- 

 ducts, the particles of which are so intimately 

 united with one another, and with those of the 

 deeper layers of the epidermis, as to allow of 

 being thus dragged out of their tube of base- 

 ment membrane, often for a length of an eighth 

 of an inch. 



The scaly epithelium is subdivisible into two 

 forms, the regular and the irregular. In the 

 former, the scales are united edge to edge in a 

 regular manner, as in the skin of the Frog and 

 other reptiles, and on many internal surfaces, 

 especially in the lower animals. In this form, 

 the particles do not become so thin as in the 

 other, and the superficial scales are cast off in 

 laminae consisting of a single series and of 

 uniform thickness. In the latter form, they 

 overlap one another without order, and present 

 no regular figure. This is the ordinary form, 

 and is that presented in the skin and other 

 parts of Mammalia and Birds. 



Of the prismatic vttritti/.* In this the par- 

 ticles have the shape of small rods, disposed 

 endwise on the basement membrane, in a single 

 layer, the thickness of which depends on their 

 length. These rods are united to one another 

 by their sides, which are flattened for that pur- 

 pose. They are, therefore, prisms and not 

 cylinders, as Henle terms them. They are abo 

 almost invariably of very unequal thickness in 

 different parts, being bulged somewhere near 

 the middle by their nucleus, which is oval, with 

 its long axis parallel to that of the particle. 

 Their deep or attached extremity, also, usually 

 tapers to a point, in order, probably, to allow 

 room for new particles to spring up in the in- 

 tervals. This is more decidedly the case where 

 they clothe a convex surface, (as that of the 



' To this the very appropriate term columnar Las 

 been lately given liy Professor Todd. 



