196 ^ EXPLANATION OF THE PLATES. 



FIG. VI. A recent specimen of the external coat of an 

 artery simply spread out upon the glass. 1, Elastic fibres ; 2, 

 fasciculi of connective fibres. 



FIG. VII. A small artery of the brain, measuring J th O f 

 a line in diameter, treated by very dilute acetic acid. 1, Ex- 

 ternal coat consisting of connective fibres ; 2, transverse mus- 

 cular cells ; 3, their nuclei ; they form the middle coat. Beneath 

 this, oval nuclei can be distinguished (4) with their long dia- 

 meters in the direction of the axis of the vessel. They are 

 imbedded in a thin layer of amorphous substance, and constitute 

 the internal coat. 



FIG. VIII. Two capillaries from the brain, the upper 

 measuring al^th and the lower 2^o tn OI> a ^ me * n diameter. 

 1, Structureless wall; 2, nuclei contained in the thickness of 

 this wall ; 3, cavity of the vessel. 



PLATE XVI. 



VEINS. 



FIG. I. Capillary, measuring T of a line in diameter ; 2, 

 minute vein, measuring ^th ; their walls are formed by con- 

 nective fibres running lengthwise of the vessel and studded 

 with numerous plasmatic cells (3). 



FIG. II. Transverse section of the femoral vein first 

 dried and then treated by acetie acid. 1-2, Internal coat; 2-3, 

 middle coat ; 3-4, external coat. In the internal coat some of 

 the elastic fibres, of which it is constituted, are seen in longi- 

 tudinal, and others in transverse section ; 5, strata of muscular 

 fibres very well characterized by their club-shaped nuclei (6), 

 the outlines of which are clear and distinct ; 7, other muscular 

 fibres, seen in transverse section, most of them having a nucleus 

 (8) ; 9, strata of elastic and connective fibres alternating with 

 the strata of muscular fibres. The external coat is similar to 

 that of the arteries. 



FIG. III. Valve from the internal Saphoena vein. 



