BLOOD-VESSELS. 



179 



single long cell may turn spirally round the tube (Lister) ; in larger 

 vessels, especially those of the arterial system, they are of course more 

 densely laid on. Outside the muscular coat is the areolar or connective 

 tissue coat, containing fibres and connective tissue corpuscles, with 

 longitudinally placed nuclei. 



In vessels of gV of an inch in diameter, or even less, the elastic layers 

 of the inner coat may be discovered (fig. 120, A, fi), in the form 



Fig. 120. 

 -/ Ap Y/f // ^ 



Fig. 120. — A Small Arterv A, with a corresponding vein B, treated with Acetio 

 Acid, and magnified 350 Diameters (after Kolliker). 



o, external coat witli elongated nuclei ; j8, nuclei of the transvei'se muscular tissue of the 

 middle coat (when seen endwise, as at the sides of the vessel, their outline is ciixular) ; 

 7, nuclei of the ejnthelium-celLs ; 5, elastic layers of the inner coat. 



generally of homogeneous or fenestrated membrane, more rarely of 

 longitudinal reticulating elastic fibres. The small veins, but two or 

 three removes from the capillaries, differ from arteries of corresponding 

 size, chiefly in the inferior development of their muscular tissue ; the 

 lining cells of the arteries also are very much longer and narrower than 

 those of the veins. These differences, as well as the comparative size of 

 corresponding vessels, are well shown in the accompanying figures. 



Termination of Arteries. — The only known termination of arteries is in veins, 

 and this takes place by means of capillary vessels as above described, unless in 

 the maternal part of the placenta and in the interior of erectile organs, in wdiich 

 it has been supposed that small arteries open into wide venous cavities without 

 the intervention of capillaries. Moreover, in the spleen, the arterial capillaries 

 do not at once unite into the commencements of the veins, but open into the 

 interstices of the organ, from which the minute veins coUect the blood. 



It is said that in certain jjarts small arteries may pass into small veins ■ndthout 

 the intervention of true capillaries (Sucquet, Hoyer). 



X 2 



