GENERAL CONSIDERATIONS. 



519 



Fiff. 268. 



Fig. 270. 



FEireSTEATED MEMBRANE 

 FROM THE CAROTID 

 ARTERY OF THE HORSE. 



Magnified 350 diame- 

 ters. 



Fig. 269. 



on the one part, and on the other with the capillaries and veins. Tor 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 epitbelium 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 

 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 

 elemcDts 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 ; 

 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 

 a layer of connective tissue, 

 with some longitudinal reticu- 

 lated elastic fibres in its deeper 

 part. Though this tunic is 

 very thin, yet it is strong ; as 



a ligature tied tightly around an artery will rupture 

 the other tunics, but not this. 



The structure of the capillaries is not the same 

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

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

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

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

 and in the largest capillaries— those immediately succeeding the small 



NETWORK or COARSE 

 ELASTIC TISSUE FROM 

 MIDDLE COAT OF PUL- 

 MONARY ARTERY OF 

 THE HORSE, THE FIBRES 

 BEING PIERCED WITH 

 CIRCULAR OPENINGS. 

 Magnified 350 diame- 

 ters. 



TRANSITION OF A MINUTE 

 ARTERY OF THE BRAIN 

 INTO CAPILLARY VES- 

 SELS. 



1, Minute artery; 2, Tran- 

 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. 



