140 



MANUAL OF HISTOLOGY. 



advancing development, the seat of various cavities which acquire subse- 

 quently on their inner surface a clothing of epithelium. It is in this 

 exceptional manner that the epithelia of serous cavities and lining mem- 

 branes of the heart, with the blood and lymphatic vessels, have had their 

 origin. 



The elements of epithelium are pale, transparent cells, with distinct 

 nuclei, only absent in the older cells of many kinds of tissues. The 

 size of these cells is liable to vary greatly; it lies between 0*0074 and 

 0'056 mm. ; that of the nucleus is less so, whose diameter may be stated 

 on an average to be from 0'0045 to 0'0091 mm. The appearance of the 

 latter may be vesicular, homogenous, or granular. 



It has already been remarked that the surfaces of the body are clothed 

 with layers of epithelium of varying thickness. The depth of the tissue, 

 in fact, changes to a most extraordinary extent in the several localities of 

 the system. Whilst some strata of epithelial cells may attain a height of 

 2 mm. and upwards upon the external skin of 

 the human body, so that they were recognisable 

 to earlier generations of anatomists without the 

 aid of the microscope, they may yet decrease 

 in thickness in other places, forming thin coat- 

 ings of only a few layers of cells, invisible to 

 the unaided eye. Finally (and this is the case 

 over by far the greatest portion of the sur- 

 face of the body), this tissue may consist 

 Fig. 127,-Fiat epithelium cells from of one sin g le extremely delicate layer of cells. 



the human mouth. The most important feature which this so 



widely-distributed tissue presents for our consideration is the variety 

 of form which it displays, which has led to the recog- 

 nition of several distinct species of epithelium. It is 

 comparatively seldom-^-and in the human body over 

 very limited areas that epithelial cells preserve the 

 original typical form of the cell, namely, the sphe- 

 roidal. We generally find either one or other of the 

 changes affecting the spherical body already considered 

 ( 46), i. e., flattening or lateral compression, so that 

 it usually appears, with modifications in particular instances, either 

 as a flattened, squamous, or narrow cylindrical cell. 



We must therefore distinguish between 1, the flattened or .pavement 

 epithelium (fig. 127) and 2, cylinder or columnar 

 epithelium (fig. 128). 



Other modifications of this tissue may arise from 

 the free surface of the cells bearing minute hair-like 

 appendages, as we have already mentioned. Thus 

 a third special form is produced, the ciliary epi- 

 thelium, fig. 129. In man and the higher animals 

 it is almost exclusively upon the cylindrical cells 

 that these supplemental structures occur. 



Again, in certain regions of the body the cell is 

 found to possess peculiar contents, namely, granules 

 of black pigment or melanin, with which its body may be charged. 



In human beings and mammalia it is only the more flattened cells of 

 the epidermis which have these exceptional contents. They represent 

 what used to be described by histologists as polyhedral pigment cells 



Fig. 128. Cylinder ojr 

 columnar epithelium 



tine of the rabbit. 



Fig. 129. Various forms of 

 ciliary cells from verte- 

 brata. 



