502 \ THE TISSUES. 



of the whole tunic, and being in a merely embryonic state of de- 

 velopment. They therefore probably manifest but little contractile 

 power. Its innermost stratum, the annular fibrous layer, contains 

 the peculiar elastic membranes or plates, of T 2oW to T^TT^ f an 

 inch, and 50 to 60 in number, regularly alternating, at distances of 

 4 ATT to 5VV f an inch, with transverse layers of smooth muscular 

 fibres, pervaded by areolar tissue. These plates are, however, not 

 mere concentric tubes, but are connected with each other, and with 

 the firm elastic network pervading the muscular tissue. (Fig. 333.) 

 In the middle coat of the medium-sized arteries the elastic plates 

 just described are absent, and the smooth muscular fibres are far 

 more abundant ; though there is here and there a disposition to the 

 formation of elastic layers, alternating with the muscular. Its 

 muscular fibres, therefore, preponderate, and doubtless manifest a 

 considerable degree of contractile force. It is thinner than the 

 external coat. 



The small arteries have their middle coat composed exclusively 

 of smooth muscular fibres; and it is stronger or weaker, according 

 to the size of the vessel, down to ^^ of an inch. In vessels of 

 yitf to ^^ of an inch, they are still united into lamella presenting 

 two or three layers, and a thickness of 2 ITJO to isV o of an inch, con- 

 stituting the annular fibrous layer next the inner coat. In smaller 

 vessels it has but a single layer of minute fibres, the latter becom- 

 ing shorter and shorter towards the capillaries. In arteries of joW 

 of an inch, embryonic forms of smooth fibres still constitute a c6n- 

 nected lamina (Fig. 337, B), but afterwards they are gradually sepa- 

 rated from each other, and become wholly lost. 



3. The inner tunic of all the arteries consists of two layers the 

 elastic and the epithelial. 1. The elastic (fenestrated, Henle) layer 

 is merely a network of longitudinal elastic fibres, resembling the 



Fig. 334. Fig. 335. 



Fig. 334. Internal or finely fibrous portion of the longitudinal fibrous (fenestrated) layer of the 

 aorta of the horse. (Magnified 200 diameters.) 



Fig. 335. External or coarsely fibrous portion of the longitudinal fibrous layer of the aorta of the 

 horse. (Magnified 200 diameters ) 



