EXPLANATION OF THE PLATES. 197 



1, Epithelium ; the oval nuclei of its cells only are visible, the 

 outlines of the cells themselves being too pale ; 2, subjacent 

 layer, formed exclusively of regularly undulating connective 

 fibres. 



FIG. IV. Elastic sub-epithelial membrane from a small 

 mesenteric vein, treated by acetic acid. 1, Web of elastic fibres ; 



2, openings of different dimensions, giving this layer the same 

 appearance as the fenestrated coat of an artery ; 3, nuclei of its 

 muscular coat, seen by transmitted light. 



FIG. Y. A mesenteric vein of yL-th of a line in diameter. 

 1, Its external coat, made up of elastic fibres, connective fibres, 

 and plasmatic cells (2) ; 3, middle coat, entirely muscular ; 4, 

 the cells which form its muscular fibres in transverse section, 

 containing nuclei ; 5, nuclei of same cells, seen in the direction of 

 their length ; 6, elastic membrane of the internal coat, rendered 

 visible by the transparency of the specimen. 



PLATE XVH. 



VEINS. LYMPHATIC VESSELS. GLANDS COMPOSED OF CLUSTERED 



FOLLICLES. 



FIG. I. Longitudinal section of the femoral vein, dried 

 and then treated by dilute acetic acid. 1, Internal coat its 

 elastic fibres almost all running parallel with the axis of the 

 vessel ; 2, middle coat ; 3, longitudinal elastic fibres ; 4, trans- 

 verse elastic fibres ; 5, muscular fibres irregularly distributed ; 

 6, their nuclei ; *7, external coat, made up of mingled elastic 

 and connective fibres ; these latter, in consequence of the action 

 of acetic acid, present the appearance of a homogeneous gra- 

 nular mass (8). 



FIG. II. Transverse section of a lymphatic vessel of 

 the thigh, treated by dilute acetic acid. Its internal coat seems 

 to be composed of but a single layer of epithelial cells. 1, 

 Middle coat, formed entirely of muscular fibres, the nuclei of 

 which are very well seen (2) ; 3, elastic fibres of which there 



