154 MANUAL OF HISTOLOGY. 



the nature of a mixture, so that its present analyses are almost worthless. 

 We may take, for instance, those quantitative ones of Mulder and Schererj 

 which apply to the epidermis of the sole of the human foot. 



(Scherer.) (Mulder.) 



C 51-036 50752 C 50-28 



H 6-801 6-761 H 6*76 



X 17-225 17-225 X 17*21 



o) 04.000 25 . 9fi9 o 25-01 



S | 24938 S 0-74 



The amount of sulphur (0'74 per cent.) in Mulder's analysis appears 

 strikingly small, while it is found to rise to between 2 and 5 per cent, in 

 the keratin of other tissues. As to the form in which it is contained in 

 the latter we know nothing. It is, however, only loosely combined. The 

 proportion of ash rises to about from 1 to 1 -5 per cent. The salts obtain- 

 able are chlorides of sodium and potassium, sulphate and phosphate of 

 calcium, phosphates of magnesium and of iron, besides which silicates are 

 also contained in the epidermis. 



The pigmentary cells possess the same characters as all the other 

 epithelial formations. Those of the eye correspond in their delicate con- 

 stitution with the non -laminated epithelia. In regard to the melanin 

 with which they are charged, comp. 37. Finally, we are still quite 

 ignorant as to what the matter is with which the nuclei of epidermal 

 cells, of dark spots of the skin, are coloured. 



95. 



The elements of epithelium stand in very close genetic relationship to 

 the gland-cells. Remak has shown that both tissues have their origin 

 from those two layers of cells continuous with one another, which clothe 

 the internal and external surfaces of the embryonic body. There like- 

 wise exists frequently between the epidermal elements and gland-cells of 

 the mature body a gradual transition : many glands, namely, are lined 

 with cells which can hardly be distinguished from those of the epithelia. 

 On the other hand, as a feature in epithelial life, the formation of 

 mucus has much in common with the destiny of certain gland ele- 

 ments; and those goblet-cells mentioned above ( 92) may be named 

 single-celled glands. Finally, the tendency which they both display to 

 excrete amorphous matter, as for instance, the thickened cell-border or 

 that which hardens into the membrana propria or basement membrane, 

 may perhaps be regarded as another feature common to the gland and 

 epidermal cells. The genesis of these two structures, however, must be 

 more fully ascertained before we can unreservedly adduce it as additional 

 proof of this relationship between the two. 



Now, when the question arises as to the purposes which epithelium 

 serves in the body, and why all the surfaces of the latter are clothed 

 with such a continuous cellular coating, we must confess ourselves in 

 a difficulty in ascribing to each species its particular properties. 



If we look for a physiological significance jn our tissue, it may be 

 said to have its basis in all probability in the relation of the latter to the 

 processes of transudation, diffusion, and absorption of the economy ; and 



