246 CIRCULATORY APPARATUS. 



§ 211. No one has ever yet seen the actual details of the 

 process bj which a capillary vessel is converted into a small 

 artery or vein. In this as in many other cases, v^e are com- 

 pelled to draw our conclusions as to what actually occurs, by 

 inference from what has gone before or what comes after. The 

 wall of a capillary vessel consists of a homogeneous, glassy 

 membrane beset at intervals with nuclei. By impregnating this 

 membrane with silver nitrate we are able to prove that it is made 

 up of plates accurately adapted to each other; to about the 

 middle of each plate a nucleus is fixed by a little soft protoplasm. 

 The plate itself may be viewed as a thin layer of hardened 

 protoplasm. The capillary membrane passes uninterruptedly 

 into a somewhat thicker, vitreous lamella which exists in all the 

 arteries and veins, even in the aorta, at the junction of the 

 internal with the middle coat ; this lamella may always be recog- 

 nised in transverse sections, not only by its greater lustre, but 

 by the uniform sinuosities of its double contour, which are due 

 to the fact that owing to its deficient elasticity, it follows tlie 

 variations in calibre of the vessel, not by alternate condensation 

 and rarefaction, but by becoming folded and unfolded (fig. 73, h). 

 On the inner surface of this membrane, the tunica intima and 

 epithelial lining are developed ; on its outer surface the muscular 

 coat and tunica adventitia. The necessary materials are fur- 

 nished by proliferation of the cells of the capillary wall, as may 

 readily be seen in the arterioles (Uebergangsgefasse). 



Fig. 78. 



Vertical section through the inner coat of the aorta. For 

 explanation see text. 5^. 



The tunica intima of the larger arteries and veins exhibits 

 the same appearances both in longitudinal and transverse sections ; 



suffices to return the blood from both areas to the heart. It is obrious 

 that such modifications cannot alter the main gist of the observations 

 made above. 



