148 MANUAL OF HISTOLOGY. 



these points may be reckoned the nipple and areola of the breast, the 

 scrotum, the labia, and the vicinity of the anus, as well as the more indi- 

 vidual cases of freckles and moles. Now, this 

 colouring, which is only found in isolated 

 portions of the bodies of those belonging to 

 the white human races, appears most exten- 

 sively in the multifarious shades of skin of 

 the remaining varieties of our species, down 

 to the deep black of many tribes of negroes. 



As *** as has hitherto been ascertained, these 

 with a eiobuie of fat lying upon darker tints (in which the fibrous tissue of 

 it; c, another in half profile. the cutis \^ affec ted) are dependent 



on three conditions, which are combined in the specially marked cases : 

 namely, on a tinging of the nucleus with a usually diffuse pigment ; 

 secondly, on a similar but much slighter colouring of the whole con- 

 tents of the cell ; and finally, on the deposit in the body of the cell of 

 a granular pigmentary matter. It is principally the deeper layers of the 

 cuticle which take part in these changes (fig. 138 b c). 



Like the mucous membranes, epidermis suffers considerable loss by 

 desquamation, owing to friction, washing, the pressure of clothes, &c., so 

 that it may be looked upon as a rather transient tissue. 



REMARKS. When these tints of the skin are less intense, we usually find that it is 

 merely the deepest and most recently formed layers of cells which contain nuclei of a 

 slightly brownish colour. But if the hue of the skin deepens, that of the nuclei be- 

 comes intensiiied to a chestnut brown or brown black. The contents, however, of the 

 cell are now no longer free, but slightly tinged with brown. Finally, in the undermost 

 layers of the cuticle we find cells with granular colouring matter also, which vary in 

 shade from yellow to brown, or even from this to the black of melanin. Here, then, 

 we have epidermal cells containing melanin also in the human subject. 



91. 



We now turn to another form of the tissue we are engaged in consider- 

 ing, known as cylinder or columnar epithelium, occurring in the human 

 body on its mucous membranes. This is the epithelium of the digestive tract, 

 whose internal surface is clothed by it from the cardiac end of the stomach 

 to the anus in uninterrupted course, where it terminates with a sharp line of 

 demarcation against the epidermis. Further, it is found in the larger excre- 

 tory ducts of those glands pouring their secretions into the intestinal tube, 

 as, for instance, those of the pancreas of the liver and gall-bladder. The 

 passages of exit likewise, from the mamma and lachrymal glands, as well as 

 certain portions of the generative system, are lined with the same cells. 

 Further, a modified cylinder-epithelium is found on isolated portions of the 

 organs of sense, as, for instance, on the regio olfactoria of the nose, and on 

 the broad papilla of the frog's tongue. We shall have to refer to this again. 

 This species of epithelium consists of a single layer or 

 coating of tall narrow cells, set up perpendicularly on their 

 ends, which either possess the same breadth through- 

 out, or are broadest at their free extremities (fig. 143), 

 while at the opposite end they are narrowed down more 

 or less to a point. In many of these cells the nucleus 

 lies aboufc in the middle, in others it is situated lower 

 the. rabbit; in pro- down. Externally, we find here also a polyhedral 

 accommodation where the cells come into contact, so 

 that cylinder epithelium, observed from above, often presents the appear- 



