49 



o 



STRUCTURE OF THE 



[sect. 214. 



the body. As for the tunica intima, the epithelial cells are not 

 usually so elongated here as in the smaller arteries, though they 

 are still fusiform and 0006'" to croi'" in size. The remaining part 

 of this coat does not necessarily become thicker with the size of 

 the vessels ; it exhibits, however, especially in the aorta, a great 



Fig. 200. 



/ / 



f> I'l'l I i ; 'S'i il'i1' 1! l' : l ,!: J ; t4 I'M. 



 ill 1 fill 



1 n| f 



1. (i n 



in-fl I l l\m\ 



1 1 .1 1 i I 4 1 u 1 LI S I U I J HI'! 



Transverse section of the aorta below the superior mesenteric artery. 1. Tunica 

 ultima; 2. tunica media ; 3. tnnica adventitia. a. Epithelium; b. striped lamella? ; c. 

 elastic coat of the tunica intima; d. elastic lamellae of the tunica media; e. muscular 

 fibres and connective tissue of the same; /. elastic networks of the tunica adventitia. 

 From the human subject. Magnified 30 times, and treated with acetic acid. 



disposition to become thickened, so that it is often difficult to 

 determine its normal thickness. With reference to its structure, 

 it chiefly consists of lamellae of a clear substance, which are some- 

 times homogeneous, sometimes striated, or even distinctly fibril- 

 lated. This has, for the most part, the characters of connective 

 tissue (Eulenburg obtained a small quantity of gelatine from the 

 tunica intima), and is traversed by finer and. coarser longitudinal 

 elastic networks. As a rule, these networks become denser and 

 denser, and their elements increase in thickness from within out- 

 wards ; and at the boundary of the tunica intima and the tunica 

 media, the inner coat presents either a dense, elastic, reticulated 

 coat, or a true fenestrated membrane, more or less fibrous, which 

 obviously corresponds to the elastic inner coat of the small arteries. 

 Immediately beneath the epithelium, the elastic fibrous networks 

 are either very fine, or are represented by one or several clear 

 layers of the striped lamellae, which, when they contain nuclei, 

 often appear to consist of coalesced epithelial cells ; but when 

 homogeneous and without nuclei, approximate rather to pale elastic 

 membranes. — In the circular fibrous coat of the larger arteries a 

 new element appears, viz., special elastic membranes or plates, 

 which, except in the transverse course of their fibres, are con- 

 structed in all essential respects like the elastic inner coat, parti- 

 cularly like that of the smaller arteries, sometimes forming very 



