STRUCTURE OF ARTERIES. 277 



inosculate on the one hand with the terminal ramusculi of the arteries ; 

 and on the other with the minute radicles of the veins. 



Arteries are composed of three coats, external, middle, and internal. 

 The external or areolo-fibrous coat is firm and 'strong, and serves at the 

 same time as the chief means of resistance of the vessel, and of connection 

 to surrounding parts. It consists of condensed areolo-fibrous tissue, 

 strengthened by an interlacement of glistening fibres which are partly 

 ongitudinal and partly encircle the cylinder of the tube in an oblique 

 direction. Upon the surface the areolar tissue is loose, to permit of the 

 movements of the artery in distention and contraction. 



The middle coat is that upon which the thickness of the artery depends; 

 it is yellowish in colour, and so brittle as to be cut through by the thread 

 in the ligature of a vessel.* 



The internal coat is a thin serous membrane which lines the interior of 

 the artery, and gives it the smooth polish which that surface presents. It 

 is continuous with the lining membrane of the heart, and through the me- 

 dium of the capillaries with that of the venous system. 



In intimate structure an artery is more complicated than the above de- 

 scription would imply. The internal coat, for example, is composed of 

 two layers, and the middle of three, so that, with the external coat, there 

 are six layers entering into the composition of an artery. The innermost 

 coat is a tesselated epithelium analogous to that of other serous membranes. 

 The second coat from within is a thin, rigid membrane, pierced with a 

 number of round or oval-shaped holes, and supporting a thin layer of flat, 

 longitudinal fibres. From these characters it has been denominated the 

 feriestrated or striated coat. The third layer, which is the innermost part 

 of the middle coat, is composed of flat, longitudinal fibres, analogous to 

 those of organic muscle. The fourth layer, the thickest of the whole, is 

 composed of muscular fibres of organic life, arranged in a circular direc- 

 tion around the vessel. The fifth, or outermost part of the middle coat, 

 is a thin layer of elastic tissue ; this is present only in the large arteries. 

 The sixth is the external or areolo-fibrous coat. 



The arteries in their distribution through the body are included in a 

 loose areolar investment which separates them from surrounding tissues, 

 and is called a sheath. Around the principal vessels the sheath is an im- 

 portant structure ; it is composed of areolo-fibrous tissue, intermingled 

 with tendinous fibres, and is continuous with the fasciae of the region in 

 which the arteries are situated, as with the thoracic and cervical fasciae in 

 the neck, transversalis and iliac fasciae, and fascia lata in the thigh, &c. 

 The sheath of the arteries contains also their accompanying veins, and 

 sometimes a nerve. 



The coats of arteries are supplied with blood like other organs of the 

 body, and the vessels which are distributed to them are named vasa vaso- 

 rum. They are also provided with nerves ; but the mode of distribution 

 of the nerves is at present unknown. 



In the consideration of the arteries, we shall first describe the aorta, and 



* The second or middle coat of the arteries has given rise to no little discussion among 

 the continental anatomists. It will be found, however, to consist of fibres, flat, elastic, 

 for the most part transverse, and belonging to the yellow elastic tissue. Some of the 

 fibres are longitudinal, and some of the transverse present strong evidences of belonging 

 to the muscular system of organic life ; so that the coat may, I think, be fairly stated to 

 be a mixed one, composed of yellow, elastic, and organic muscular fibres. *l is beat 

 studied in tre aorta or some large trunk. G. 



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