49 2 STRUCTURE OF VEINS. [sect. 215. 



cular fibre-cells of other organs ; yet even here they retain a some- 

 what stiff and peculiar appearance. In the carotid, axillary, iliac, 

 and subclavian arteries, the contractile elements are more deve- 

 loped, and, accordingly, the middle coat of these vessels has not 

 the pure yellow colour of that of the largest arteries, but more of 

 a reddish tinge. — The tunica adventitia of the largest arteries is 

 thinner, both relatively and absolutely, than that of the smaller, 

 amounting to only 004'" to o'02'". Its structure is, upon the 

 whole, the same as in other arteries, yet its inner elastic layer is 

 much less developed, and, on account of the thick elastic elements 

 in the tunica media, it is but little marked-off from the latter 

 coat. 



The tunica intima, also, of certain arteries contains smooth muscular fibres, 

 as I have found in the axillaries and popliteals of man, and as Remak has also 

 lately demonstrated in the visceral arteries of mammalian animals. In the 

 large arteries of man, this coat is very frequently thickened, and then a great 

 increase of the striped lamellse more especially takes place. — In the tunica 

 media, muscular fibres are not entirely absent in any artery ; but they are 

 wanting on the arterial branches of the retina, which measure less than o'o2'". 

 — The tunica adventitia of large arteries contains muscular fibres in animals, 

 but none in man. According to J. Lister (Trans, of Royal Society of Udin., 

 1857, and Quart. Journ. of Micr. Sc, Oct., 1857, p. 8), the smallest arteries of 

 the frog's web show contractile fibre-cells, which measure from ^ 5 to ^a of an 

 inch, and run in a spiral direction, making one-and-a-half up to two-and-a-half 

 turns round the inner coat of the vessel ; and such fibre-cells in a single 

 layer constitute the only muscular elements of the vessel. 



§ 215. Veins. — The veins, also, may be divided into three 

 groups, small, medium-sized, and large; but these groups cannot 

 be so definitely separated from one another, as was the case in the 

 arteries. The veins invariably possess thinner walls than the 

 arteries, and this is dependent upon the smaller amount of their 

 contractile elements, as well as upon the more scanty development 

 of their elastic parts; and hence the veins are more flaccid and 

 less contractile. — The tunica intima is frequently of the same 

 thickness in large as in middle-sized veins; it is less developed 

 than in the arteries, but, in all other essential points, is similarly 

 constructed. The tunica media, which is never yellow, but mostly 

 reddish-grey in colour, contains much more connective tissue, and 

 fewer elastic and muscular fibres, and it always presents the very 

 distinguishing feature of laminse running longitudinally as well as 

 transversely. This coat is usually thin, though, in the middle- 

 sized veins, it is even absolutely thicker than in the large, and, in 

 such veins, it is best provided with muscular fibres. — The tunica 



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