CHAP, iv.] THE VASCULAR MECHANISM. 191 



which, except for a remnant of undifferentiated protoplasm round 

 the nucleus, has become converted into transparent, differentiated 

 material. Since the cells, except for the minimum of cement 

 substance between them, are in close contact with each other, we 

 might speak of them as forming an epithelium ; but on account 

 of their cell body being reduced to a mere plate, and on account of 

 their connection both by origin and nature, with mesoblastic 

 connective tissue corpuscles, it is convenient to speak of them as 

 epithelioid cells or plates. They are sometimes spoken of as 

 endothelial cells or plates. In a small capillary the width of one 

 of these epithelioid plates at its widest part, where the nucleus 

 lies, may be of nearly the same size as the circumference of the 

 even distended capillary ; the cells consequently are placed not 

 side by side, but more or less alternate with each other, and their 

 nuclei project alternately into the lumen of the vessel. The 

 larger capillaries may, however, be so wide that two or even more 

 cells lie more or less abreast. Outside the capillary, which 

 is thus a thin and delicate membrane, a mere patchwork of 

 thin epithelioid cells cemented together, is always found a certain 

 amount of connective tissue, the wall of the capillary forming at 

 places part of the walls of the lymph-holding connective tissue 

 spaces, and at other places being united by cement material to the 

 bundles, bands or sheets of the same connective tissue. Not un- 

 frequently, in young tissues, branched connective tissue corpuscles 

 lie upon and embrace a capillary, some of the processes of the cell 

 being attached to the outside of the epithelioid plates of the capil- 

 lary. Even in the capillaries of such a tissue as muscle, the net- 

 work of capillaries embracing a muscular fibre is always surrounded 

 by a certain, though sometimes a small amount only, of connective 

 tissue ; indeed wherever capillaries run they are accompanied 

 as we have said by connective tissue, so that everywhere all over 

 the body, the blood in the capillary is separated from the lymph 

 in the spaces of the connective tissue by nothing more than the 

 exceedingly thin bodies of the cemented epithelioid plates. It must 

 be added, however, that the spaces in the connective tissue are 

 themselves sometimes lined by similar epithelioid plates, of which 

 we shall have to treat in speaking of the lymphatics, so that in 

 places the partition between the blood and these lymph spaces 

 may be a double one, and consist of two layers of thin plates. 



In any case, however, the partition is an exceedingly thin one, 

 and so permeable that it allows an adequately rapid interchange 

 of material between the blood and the lymph. As we shall 

 presently see, not only fluids, that is, matters in solution, are able 

 to pass through the partition into the lymph, but intact corpuscles 

 both red and white, especially the latter, may, in certain cir- 

 cumstances, make their way through, and so pass from the interior 

 of the capillary into the lymph spaces outside. It is probable, 

 however, that these make their way chiefly, if not exclusively, 



