142 



MANUAL OF HISTOLOGY. 



Fig. 133. Simple pavement epi- 

 thelia. Serous membrane at, a ; 

 from a vessel at b with lateral 

 view. 



The elements of this tissue consist of pale flat cells (fig. 133) placed 

 closely together, and without any apparent intercellular matter. They are 

 frequently destitute of granular contents, but display at times very minute 

 dust-like molecules. Such is the indistinctness of limitation in the cells, in 

 certain instances, that their outlines may not be 

 apparent, or they may seem to fuse into one an- 

 other. Their boundaries usually become visible, 

 however, in the form of dark lines, on treat- 

 ment with a dilute solution of nitrate of silver. 



These cells possess distinct nuclei, sometimes 

 granular, sometimes smooth-edged, in the in- 

 terior of which one or more nucleoli are usually 

 visible. Their form is twofold; in one instance 

 they may be broad, with a polyhedral outline 

 (d) and diameter of 0'0226-0'0090 mm., while 

 the round nucleus is 0'0075-0'0057 mm. ; in another their shape is more 

 or less lanceolate, and length '0226-0 '0455 mm. with a similarly nar- 

 rowed nucleus (b). In side view such cells may present a very peculiar 

 appearance (&); they then seem to possess the form of short fibres, thickened 

 considerably in the middle, where the nucleus is situated. The first of 

 these species of cells is found lining serous sacs, the latter clothing the 

 internal surface of blood-vessels and lymphatics ; but here again there 

 exists much variety (Legros). In the arteries we find long and narrow 

 cells, while the endothelium of veins is made up of shorter and broader 

 elements. 



The thickness of these structures, and with it that of the whole covering, 

 must, as we have already mentioned, present much variety. Where 

 but a small amount of flattening has taken place, the depth of the cell 

 and thickness of the whole layer is generally about 0'0055 mm. and 

 upwards, whilst strata which have undergone more compression may 

 sink in depth to only -0037-0 -0032 mm. 



Those tall cells, again, which occur in the hollows of the brain also 



deserve special notice as peculiar elements ; 

 also those of the choroid plexus. The latter 

 (fig. 134) are likewise thicker and rounder, 

 giving off one or more pointed processes, 

 and containing, besides the nucleus, as a 

 rule, one or several granules of a dark brown 

 substance, which is, moreover, absent in 

 the younger cells. 



Pavement epithelia are very delicate 

 structures which undergo rapid decay in the 

 dead body. In the living subject, on the other hand, they probably con- 

 stitute more durable tissues, with but little power of rapid regeneration 

 however. The epithelia of the lung are perhaps an exception in this 

 respect. The mode of their regeneration is not yet known. 



REMARKS.!. See ffcnle's Allg. Anat. p. 226, &c. Luschka, Die Structur der serdsen 

 Haute. Tubingen, 1851. 2. We refer our readers to the special chapter on the vas- 

 cular system for the rest. 3. Those views which were formerly entertained on this 

 point, and which taught of the existence of a laminated pavement epithelium on the 

 surface of connective tissue, were based on deceptive appearances. However, at an 

 early embryonic period the surfaces of cartilages do seem to be clothed with a layer 

 of cells similar to those of epithelium. We will refer to this again. 4. Comp. the 

 paragraph on the respiratory apparatus for the epithelium in question. 



Fig. 134. Epithelial cells from the hu- 

 man choroid plexus, a, cells from 

 above; 6 and c, side views of the same. 



