CAPILLARY CIRCULATION. G89 



tions, three layers, which are designated as the inner coat, 

 or tunica intima ; the middle coat or circular hbrous coat, 

 tunica riiedia; and an outer coat, tunica externa or ad- 

 ventitia. The first is the thinnest coat, and consists of a 

 cellular layer, the epithelium ; generally, also of an elastic 

 coat, with the fibres disposed in a longitudinal direction. 

 The second, middle coat, is a thicker layer, and the principal 

 seat of the muscular fibres of the vessels ; in the veins, 

 however, it contains numerous longitudinal fibres, and in 

 the largest vessels more or less elastic elements and con- 

 nective tissue. The third coat has its fibres again arranged 

 for the most part longitudinally, and it is as thick as, or even 

 thicker than, the middle coat, and consists of connective 

 and elastic tissues. This coat, the tunica adventitia of 

 large arteries, contains muscular fibres in animals, but none 

 in man. According to J. Lister {(Jiiart. Journ, Mici^os. 

 Scien. 1857, p. 8), the smallest arteries of the frog's web 

 show contractile fibre-cells, which measure from the 1-lOOth 

 to l-200th 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. 



The blood-vessels of the eye are extremely numerous, 

 and present difterent conditions in the several parts. In 

 the choroid coat, they are arranged in a most beautiful 

 stellate manner, and the capillary network of the inner- 

 most layer of the choroid coat, when injected, forms an 

 attractive object. A vertical section of the eye of a cat, 

 showing its several vascular and nervous coats, is given in 

 Plate VII. No. 157. 



The circulation in the foot of the Frog and the tail 

 of the i^ewt is, for the most part, the capillary circula- 

 tion. The ramifications of the minute arteries form a 

 continuous network, from which the small branches of 

 the veins take their rise The point at which the arteries 

 terminate and the minute veins commence, cannot be 

 exactly defined ; the transition is gradual ; but the inter- 

 mediate network is so far peculiar, that the small vessels 

 which compose it maintain nearly the same size through- 

 out ; they do not diminish in diameter in one direction, 

 like arteries and veins ; hence the term capillary, from 



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