SECT. 214.] STRUCTURE OF SMALLEST ARTERIES. 



487 



always exhibits more or less large longitudinal plication, and fre- 

 quently also numerous fine transverse folds, which (although it is per- 

 fectly homogeneous) give to this membrane a peculiar longitudinal 

 striated appearance. Moreover, it almost always appears as a so- 

 called fenestrated membrane, with well marked reticulated fibres 

 of various sizes, and openings mostly small and elongated ; more 

 rarely, it has the structure of a true but very dense network of 

 longitudinal elastic fibres, with narrow elongated fissures, and then 

 completely agrees with the elastic larnelke of the middle coat of 

 the larger arteries, in its appearance, in its great elasticity, and in 

 its chemical reactions. — The middle coat of small arteries, such as 

 we are now considering, is purely muscular, without the slightest 

 intermixture of connective tissue or elastic elements, and varies in 

 thickness according to the size of the arteries (down to o , 03'"). 

 Its fibre-cells, united to form lamellce, can be pretty easily isolated 

 by dissection in vessels not larger than one-tenth of a line, and in 

 still smaller vessels they may be found by boiling and maceration 

 in nitric acid of 20 per cent., and they are then seen to be beautiful 

 fibre-cells, 0-02'" to 0-03'" long, and 0-002'" to 0-0025'" broad. — 

 The tunica adventitia consists of connective tissue and fine elastic 

 fibres, and is generally as thick, or even thicker than the tunica 

 media. 



The structure just described holds good for arteries as small as 

 £'" in diameter, but further on towards 

 the capillaries it changes more and 

 more (fig. 198). Even on arteries ^5", 

 the adventitia no longer contains elastic 

 fibrils, but onlv connective tissue with 

 elongated nuclei, which, at first, is 

 fibrous, but gradually becomes more 

 and more homogeneous, retaining its 

 nuclei, and, at last, is reduced to a thin, 

 perfectly homogeneous envelope, which 

 entirely disappears on vessels below 

 0-007'". — The cn ' cu l ar fibrous coat, on 

 arteries below -^~" down to those of 

 J3-'", still possesses two or three layers 

 of muscular fibres, and a thickness of 

 0-008'" to 0-005'"; ou smaller arteries, 

 it forms only one layer, whose elements, 

 at the same time, become shorter and 

 shorter, and, at last, on vessels between 



a. An artery 001'", and b. a vein 

 - 015'",from the mesentery of a child, 

 magnified 350 times, and treated with 

 acetic acid. The letters as in the 

 previous figure ; <?. the tunica media 

 of the vein, consisting of nucleated 

 connective tissue. 



