142 



LECTURE VI. 



stance of the circular-fibre coat, but only at the border of 

 the vessel (Figs. 26, , b ; 46 ,m, m\ and which look like 

 round cells or nuclei scattered through the tissue. These 

 are fibre-cells seen in apparent transverse section. The 

 layer formed by the middle coat may be most distinctly 

 seen, however, after the addition of acetic acid, which 

 causes the appearance of a great number of oblong 

 nuclei. 



It is this layer which, generally speaking, confers upon 

 the arteries their specific character, and distinguishes 

 them most clearly from the veins. There are, indeed, 

 numerous veins in the body which possess considerable 

 layers of muscular tissue for example, the superficial 

 cutaneous veins ; still, in the case of the smaller vessels, 

 the occurrence of a distinctly marked circular-fibre coat 



Fig. 46. 



is so peculiarly characteristic of arterial vessels, that, 

 wherever we meet with such a structure, we are at once 

 inclined to assume the vessel to be arterial. 



These vessels, which must be included among the 



Fig. 46. A minute artery from the sheath of the tendon of one of the extensors 

 of a hand just amputated, a, a. External coat, m, m. Middle coat, with well- 

 developed muscular layer. ', i. Internal coat, partly with longitudinal folds, partly 

 with longitudinal nuclei, in the side-branch brought well into view in consequence 

 of the two external coats having, been torn away. 300 diameters. 



