GENERAL CONSIDERATIONS. 



519 



. 26g 



Fi S- 27 - 



TU & A 

 Maerninea 



ter s. 



A- 

 diame- 



on the one part, and on the other with the capillaries and veins. For a 

 long time it has been assimilated to a serous membrane, but it has not 

 absolutely the same texture. It is composed of a simple epithelial layer 

 which is in contact with the blood, and is formed by fusiform cells that 

 slightly bulge in the situation of their nucleus. 

 These cells sometimes become detached, and are 

 carried about in the nutritive fluid, in which, after a 

 certain period, they resemble 

 more or less mis-shapen blood- 

 globules. The epithelium lies 

 upon a layer of amorphous 

 elastic tissue, perforated by 

 openings, and named the fene- 

 strated membrane ; on its ex- 

 ternal face are proper elastic 

 fibres passing in a longitudinal 



FENESTRATED MEMBRANE direction. 



The middle tunic is remark- 

 able for its thickness, its elas- 

 ticity, and the yellow colour it 

 offers in the principal vessels. 



It is composed of a mixture of elastic fibres, as well as 



smooth muscular fibres, the first constituting a kind of 



network, in the meshes of which the contractile fibres 



are disposed in a circular manner around the vessels. 

 The proportion of these two 

 elements varies with the size 

 and situation of the artery. 

 In the large trunks, such as 

 the aorta, the elastic is more 



abundant than the contractile ; 1, Minute artery; 2,Tran- 

 in the middle-sized vessels 

 they are about equal ; but in 

 the small arteries, in which 

 the contractile force of the 

 heart is lost because of their 

 distance from it, the muscular 

 fibres almost exclusively com- 

 pose the middle tunic. 



The external tunic is only 

 FROM a l aver f connective tissue, 

 MIDDLE COAT OF PUL- with some longitudinal reticu- 

 MONARY ARTERY OF lated elastic fibres in its deeper 



THE HORSE, THE FIBRES part Though this tunic IS 



cmcuLAR IERC op > ENmGs I verv thin, yet it is strong ; as 



Magnified 350 diame- a ligature tied tightly around an artery will rupture 



ters. the other tunics, but not this. 



The structure of the capillaries is not the same 



as that just described, but is modified in proportion as they are fine. In 

 the smallest capillaries, the walls are formed by a thin amorphous mem- 

 brane, in which (oblong) nuclei are somewhat regularly disseminated; in 

 medium-sized vessels, another layer containing transverse nuclei is observed ; 

 and in the largest capillaries those immediately succeeding the small 



Fig. 269. 



OF COARSE 

 TISSUE 



sitional capillary ; 3, 

 Coarse capillary with 

 thick coat, represented 

 by a double contour 

 line ; 4, Fine capillary, 

 with single contour. 

 The nuclei are seen 

 widely scattered in 4 

 and 3 ; more closely con- 

 gregated in 2 ; and still 

 more so in 1, where they 

 form an epithelium. a, 

 Transverse elongated 

 nuclei of muscular cells, 

 the incipient muscular 

 coat of the artery. 



