TISSUES OF THE BODY. 



363 



parts of the body, are tubes just sufficiently large to permit the passage 

 through them of a single blood corpuscle, which can even be compressed in 

 its course. The diameter of the lumen may be stated consequently at 

 -0045-0 '0068 mm., whilst other and more considerable tubes attain a 

 breadth of 0'0113 mm. and upwards. 



These canals (355, A, B) were supposed, until very recently, to have 

 an extremely simple texture. As a rule, their walls are perfectly trans- 

 parent and structureless, and endowed with remarkable elasticity and 

 extensibility. Chemically, they resemble the sarcolemma of muscle 

 fibres and primitive sheath of nerves, displaying a considerable power 

 of resisting the action of many strong reagents. In the walls of these 

 tubes rounded or oval nuclei are to be seen, 0'0056--0'0074 mm. in 

 diameter, in which nucleoli may be remarked. These are arranged 

 irregularly one behind the other at considerable intervals (A, a, b, J3, a), 

 but at times at more regular distances (A, a, B, b). In larger branches, 

 measuring perhaps 0'0113 mm. and upwards, the latter arrangement is 

 the rule; otherwise the structure remains the same, except that such 

 tubes may attain considerable thickness, amounting to 0'0018 mm. The 

 long axis of the nuclei corresponds to that of the vessel ; they are conse- 

 quently said to be longitudinally oval in figure. 



202. 



This view just mentioned of the nature of the walls of capillaries was 

 held for many years with unquestioning tenacity, no expedient having as 

 yet been hit upon by which the structure of the trans- 

 parent nucleated membrane of which they consist could 

 be farther resolved. 



However, all at once the analysis of structure was 

 accomplished through the discoveries of Auerbach, 

 Eberth, and Aeby following in the footsteps of Hoyer. 

 From them we learned the usefulness of very dilute 

 solutions of nitrate of silver in rendering visible, in 

 the most exquisite manner, the delicate contour of 

 cells (whether those of epithelium or smooth muscle) 

 in the form of dark lines. The transpare^ nucleated 

 membrane in question is formed of flat cells, often 

 peculiarly bordered, and having a single nucleus (figs. 

 356 and 357); they are united closely with one 

 another at their edges, and curved towards the lumen 

 of the vessel. The tube thus formed is endowed, 

 moreover, with vital contractility (Strieker}. 



These cells, further, extend continuously into the 

 more considerable and even the largest trunks, though 

 to a certain extent modified. This may easily be re- 

 cognised. Here they were known even to the earlier 

 histologists, their contour being plainly visible without 

 any further treatment. They were described as the 

 epithelia of the arteries, veins, and cardiac cavities 

 ( 87), and we may add, with perfect correctness, for 

 these lining cells of the vascular system are members 

 of the epithelium group of the middle germinal layer (pp. 158, 159), the 

 endothelia of His. Another name has been proposed for them also by 

 Auerbach, namely, perithelium. It may be found more convenient, how- 



Fig. 356. Capillaries 

 from the mesentery 

 of a Guinea-pig after 

 treatment with solu- 

 tion of nitrate of sil- 

 ver, a, cells; 6, nu- 

 clei of the same. 



