THE CIRCULATORY SYSTEM. 



117 



the vessel is contracted they are thicker and the portion containing 

 the nucleus projects slightly into the lumen of the vessel. 



The subendothelial fibrous tissue forming the second layer of the 

 intima is composed of very delicate fibrils, closely packed together, 

 with a little cement between them, and enclosing irregular spaces in 

 which the branching cells of the tissue lie. Elastic fibres, spring- 



Fig. 103. 







>^Si 



KX 



mm 









Branch of splenic artery of a rabbit : o, internal endothelial surface of the intima ; b, 

 elastic lamina of the intima (fenestrated membrane, see Fig. 61) ; c, media composed 

 of smooth muscular tissue encircling the vessel and therefore appearing in longitudinal 

 section with elongated nuclei ; d, adventitia of fibrous tissue blending above and to the 

 left with the surrounding areolar tissue ; e. adipose tissue, between the cells of which a 

 few lines of red corpuscles reveal the presence of capillary bloodvessels; /, small nerve, 

 containing both medullated and pale or non-medullated nerve-fibres. There are other 

 similar sections of nerves in the figure. To the left of the artery the section is slightly 

 torn, the adipose tissue being separated from the adventitia of the artery. A few red 

 blood-corpuscles have been extravasated near the nerve at the upper left corner of the 

 figure. There are also a few corpuscles within the lumen of the artery. 



ing from the external layer of the intima, may here and there, 

 especially in the larger arteries, make their way into the subendo- 

 thelial layer. 



The elastic lamina of the intima is formed by a network of anas- 

 tomosing elastic fibres, having a general longitudinal disposition with 

 respect to the axis of the vessel. The spaces left between the fibres 

 of this network vary considerably in size. Where they are small 

 and the fibres between them are correspondingly broad this layer 

 has the appearance of a perforated membrane (the fenestrated mem- 

 brane of Henle). Even where this membranous character of the 

 elastic layer is well developed, elastic fibres are given off from its 



