STRUCTURE OF ARTERIES. 361 



When arteries unite they are said to anastomose or inosculate. Anastomoses 

 may occur in tolerably large arteries, as those at the base of the brain, those of the 

 hand and foot, and the mesentery, but they are much more frequent in the smaller 

 vessels. Such inosculations admit of a free communication between the currents of 

 blood, and must tend to promote equability of distribution and of pressure, and to 

 obviate the effects of local interruption. 



Arteries commonly pursue a tolerably straight course, but in some parts they are 

 tortuous. Examples of this in the human body are afforded by the arteries of the 

 lips and of the uterus, but more striking instances may be seen in some of the lower 

 animals, as in the well-known case of the long and tortuous spermatic arteries of the 

 ram and the bull. In very moveable parts like the lips, this tortuosity will allow 

 the vessel to follow their motions without undue stretching ; but in other cases its 

 purpose is not clear. The physical effect of such a condition of the vessel on the 

 blood flowing along it must be to reduce the velocity, by increasing the extent of 

 surfa.ce over which the blood moves, and consequently the amount of impediment 

 from friction ; still it does not satisfactorily appear why such an end should be 

 provided for in the several cases in which arteries are known to follow a tortuous 

 course. The same remark applies to the peculiar arrangement of vessels named a 

 " rete mirabile," where an artery suddenly divides into small anastomosing branches, 

 which in many cases unite again to re-construct and continue the trunk. Of such 

 rctia mirabilia there are many examples in the lower animals, but, as already 

 remarked, the purpose which they serve is not apparent. The best known instance 

 is that named the rete mirabile of Galen, which is formed by the intracranial part of 

 the internal carotid artery of the sheep and several other quadrupeds. 



Arteries possess considerable strength and a very high degree of elasticity, being 

 extensible and retractile both in their length and their width. When cut across 

 they present, although empty, an open orifice ; the veins, on the other hand, 

 collapse, unless when prevented by connection with surrounding rigid parts. 



Structure. In most parts of the body the arteries are inclosed in a sheath 

 formed of connective tissue, and their outer coat is connected to the sheath by fila- 

 ments of the same tissue, but so loosely that, when the vessel is cut across, its ends 



Fig. 417. TRANSVERSE SECTION OF PART OF THE WALL OF THE POSTERIOR TIBIAL ARTERY (MAN). 



75 DIAMETERS. (E. A. S. ) 



a, epithelial (endothelial) and subepithelial layers of inner coat ; b, elastic layer (fencstrated 

 membrane), of inner coat, appearing as a bright line in section ; c, muscular layer (middle coat) ; 

 d, outer coat, consisting of connective tissue bundles. In the interstices of the bundles are some 

 connective tissue nuclei, and, especially near the muscular coat, a number of elastic fibres cut across. 



readily shrink some way within the sheath. Some arteries lack sheaths, those for 

 example which are situated within the cavity of the cranium. 



Independently of this sheath, arteries (except those of minute size whose 

 structure will be afterwards noticed) have been usually described as formed of three 

 coats, named, from their relative position, internal, middle, and external (fig. 417, in 

 section) ; and as this nomenclature is generally followed in medical and surgical 

 works, and also correctly applies to the structure of arteries so far as it is discernible 



