GENERAL ANATOMY. 359 



twigs, these inosculations become so numerous as to constitute a close network 

 that pervades nearly every tissue of the body. 



Throughout the body generally, the larger arterial branches pursue a perfectly 

 straight course, but in certain situations they are tortuous : thus, the facial artery 

 in its course over the face, and the labial arteries of the lips, are extremely tor- 

 tuous in their course, to accommodate themselves to the movements of these parts. 

 The uterine arteries are also tortuous, to accommodate themselves to the increase 

 of size which this organ undergoes during pregnancy. Again, the internal carotid 

 and vertebral arteries, previous to their entering the cavity of the skull, describe 

 a series of curves, which are evidently intended to diminish the velocity of the 

 current of blood, by increasing the extent of surface over which it moves, and 

 adding to the amount of impediment which is produced from friction. 



The arteries are dense in structure, of considerable strength, highly elastic, and, 

 when divided, they preserve, although empty, 'their cylindrical form. 



The arteries are composed of three coats internal, middle, and external. 



The internal, the thinnest, consists usually of two layers, an inner or epithelial, and 

 an outer or elastic coat. The former consists of a single layer of fusiform-shaped 

 epithelial cells with round or oval nuclei. The latter is a transparent, colorless, 

 shining membrane, perforated with small elongated apertures (hence the name, 

 fenestrated}, and marked with numerous reticulations. This layer is perfectly 

 smooth when the artery is distended ; but when empty presents numerous longi- 

 tudinal and transverse folds. 



In arteries above the size of the capillaries, the elastic layer is very delicate, 

 and the epithelium clearly demonstrable. 



In arteries of less than a line in diameter, the internal coat consists of two layers, 

 as above described ; but in medium-sized arteries, several lamellae, composed of 

 elastic fibres and connective tissue, are interposed between the epithelial and elastic 

 coats. In the largest arteries, the inner coat is usually much thickened, especially 

 in the aorta ; and consists of a homogeneous substance, occasionally striated or 

 fibrillated, traversed by longitudinal elastic networks, which are very fine in the 

 lamellae immediately beneath the epithelium, but increase in thickness from within 

 outwards. The internal and middle coats are separated, by either a dense elastic 

 reticulated coat or a true fenestrated membrane. 



The middle coat, thicker than the preceding, consists of muscular and elastic 

 fibres, and connective tissue, disposed chiefly in the transverse direction. In the 

 largest arteries, this coat is of great thickness, of a yellow color, and highly 

 elastic ; it diminishes in thickness, and becomes redder in color as the arteries 

 become smaller, becomes very thin, and finally disappears. In small arteries, this 

 coat is purely muscular, consisting of muscular fibre-cells united to form lamellae, 

 which vary in number according to the size of the arteries, the very small arteries 

 having only a single layer, and those not larger than the T Vth of a line three or 

 four layers. In arteries of medium size, this coat becomes thicker in proportion 

 with the size of the vessel ; its layers of muscular tissue are more numerous, and 

 intermixed with numerous fine elastic fibres which unite to form broad-meshed 

 networks. In the larger vessels of this class, as the femoral, superior mesenteric, 

 cceliac, external iliac, brachial, and popliteal arteries, the elastic fibres unite to form 

 lamellae, which alternate with the layers of muscular fibre. In the largest arteries, 

 the muscular tissue is only slightly developed, and forms about one-third or one- 

 fourth of the whole substance of the middle coat. This is especially the case in the 

 aorta, and trunk of the pulmonary artery, in which the individual cells of the 

 muscular layer are imperfectly formed ; while, in the carotid, axillary, iliac, and 

 subclavian arteries, the muscular tissue of the middle coat is more developed. 

 The elastic lamellae are well marked, may amount to fifty or sixty in number, and 

 alternate regularly with the layers of muscular fibre. They are most distinct, and 

 arranged with most regularity, in the abdominal aorta, innominate artery, and 

 common carotid. 



The external, or areolar and elastic coat, the thickest of the three, consists of 



