238 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



very positive account can be given, of the mode in which the epithelial cells 

 originate, but there are appearances which indicate that it is not always the 

 same. Thus, in most of the cases in which a spheroidal epithelium presents 

 itself as the active instrument of secretion (as, for instance, on the villous 

 prominences of the synovial membranes ( 227), its cells are observed to be in 

 different stages of development, and they are embedded in a granular blastema, 

 in which it appears probable that they may originate de novo, after the manner 

 already described ( 106). But in many other cases, the epithelial cells cover- 

 ing an extensive tract are so very similar to one another in form, size, and grade of 

 development (Fig. 25), that it is obvious that they must have been all produced, 

 and all brought to a readiness for exuviation, at the same time ; and it not 

 unfrequently happens, more especially in the case of the cylinder epithelium of the 

 intestinal villi, that, when detached from the basement membrane, the cells are 

 still found to be adherent to each other, and to carry upon their broad free sur- 

 face a thin membranous pellicle (a, a), which may be made more distinct by 

 the action of water. Here it would seem more likely that the cells are de- 

 veloped in the very substance of the basement membrane, perhaps from a germi- 

 nal spots" contained in itself, as suggested by Prof. Groodsir ( 116) ; and that, 

 in the course of their enlargement, they carry before them the outer layer of 

 the basement membrane beneath which they originated. So, again, in the case 

 of the secreting follicles, there are indications that the cells they contain are 

 sometimes developed in the midst of a blastema exuded from their walls; 

 whilst in other instances the origin of the cells seems traceable to a " germinal 

 spot" at the caecal extremity of the follicle, or to several such spots dispersed 

 over its sides. Upon the whole of this subject the conditions under which 

 the exuviation of the Epithelium occurs, the frequency with which it usually 

 takes place, and the mode in which its renewal is effected much still remains 

 to be learned. 



234. We have now to consider, in somewhat mofe detail, certain appendages 

 to the Mucous membranes, which are found in connection with particular parts 

 of them ; and which may be considered as special developments of their ordi- 

 nary elements. Thus in the mouth, and especially on the tongue, we meet with 

 numerous slight elevations or papillae, some of which are very minute and 

 simple, whilst others are larger and more complex, being cleft (as it were) 

 into secondary papillae. The intimate structure of these is by no means 

 uniform; and the purposes which they answer are probably very diverse. 

 Thus, whilst the "fungiform" papillae have a soft epithelial covering, and 

 are copiously supplied with bloodvessels and nerves, so as to serve for the re- 

 ception of gustative impressions, the " conical" are furnished with a firm horny 

 epithelial investment, sometimes prolonged into fine filaments, and are less copi- 

 ously supplied with nerves and bloodvessels, their function being probably the 

 purely mechanical one of assisting in the abrasion and comminution of the food. 

 It is curious that the fungiform papillae contain striated muscular fibres, which 

 pass up to them from the muscular substance of the tongue, a fact first an- 

 nounced, in regard to the Frog, by Dr. Edmund Waller; 1 and that they under- 

 go a kind of erection from the turgescence of their vessels, when sapid sub- 

 stances are brought into contact with them. In the oesophagus and stomach, 

 we find the mucous membrane usually lying in rugae or wrinkles, which are dis- 

 posed with some regularity ; these, however, are simple folds into which the 

 membrane is thrown by the contraction of the muscular coats of these organs, 

 and are obliterated by distension of the latter. A permanent series of folds, 

 however, which can only be obliterated by dividing the outer coats of the canal, 

 are found in the small intestine, where they are known as the "valvulae conni- 

 ventes." The chief use of these appears to be to increase the absorbent surface. 



1 "Philosophical Transactions," 1849, p. 143. 



