200 GENKKAL ANATOMY. 



279. The origin of the mucous membrane, from the very 

 beginning of the egg and its development in the embryo, have 

 been already pointed out. (256.) "There yet remains unde- 

 scribed the formation of the villosities; it is to M. Fr. Meckel 

 that we are indebted for our knowledge of this point of em- 

 bryogeny. The villosities are formed at a very early period. 

 From the beginning of the third month, they are visible in the 

 form of closely-joined, longitudinal plaits. These plaits after- 

 wards present, on their free edge, notches like the teeth of a 

 saw, which successively augment in depth; and towards the 

 end of the fourth month, the plaits are replaced by that multi- 

 tude of little eminences which constitute the villosities. They 

 are at first tolerably large and very distinct till the seventh 

 month. In the commencement they are as numerous, although 

 shorter, in the large intestine, as in the small one. Those of 

 the large intestine afterwards diminish in number till birth. 

 We should observe that in reptiles, these villosities are re- 

 placed by little longitudinal folds. 



280. The differences of the mucous membrane, as regards 

 the sexes, races, and individuals, are not such as can be gene- 

 rally described, excepting always the difference of conforma- 

 tion in the genital and urinary organs of the two sexes. The 

 mucous membrane of the digestive canal, is thicker in the hu- 

 man species than in the mammiferous carnivora, but thinner 

 than in the herbivora; the peritoneal covering of the intestine, 

 on the contrary, is thinner in the herbivora, and thicker in the 

 carnivora than in man. 



281. The teeth, as has been already stated, are appendages 

 of the mucous membrane of the mouth, -prolonged into the al- 

 veoli, as far as the papilla or dental pulp, appendages which 

 may be compared to the hairy appendages and horns of the 

 external skin. 



282. The mucous membrane is subject to extremely varied 

 and numerous morbid alterations: it participates in the primi- 

 tive and acquired vices of conformation, of the organs of which 

 it is a part, as well as of their displacements. It alone also 

 undergoes displacements, more or less extensive, through the 

 loosened texture of the sub-mucous tissue, particularly in the 



