clxviii BLOOD-VESSELS. 



moses may occur in tolerably large arteries, as those of the brain, 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 move- 

 able 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 

 surface over which the blood moves, and consequently the amount of im- 

 pediment 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 reconstruct and continue the trunk. Of such retia mirabilia there are 

 many examples in the lower animals, but, as already remarked, the pur- 

 pose which they serve is not apparent. The best known instance is that 

 named the rtte mirabile of Galen, which is formed by the intracranial 

 part of the internal carotid artery of the sheep and several other quad- 

 rupeds. 



Physical Properties. 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 filaments of the same tissue, but so loosely that when the vessel 

 is cut across its ends readily shrink some way within the sheath. The 

 sheath may inclose other parts along with the artery, as in the case of that 

 enveloping the carotid artery, which also includes the internal jugular vein 

 and pneumo-gastric nerve. Some arteries want sheaths, as those for ex- 

 ample which are situated within the cavity of the cranium. 



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

 structure will be afterwards described with that of the capillaries) have 

 been usually described as formed of three coats, named, from their rela- 

 tive position, internal, middle, and external ; 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 by the naked eye, it 

 seems best to adhere to it as the basis of our description ; although it will 

 be seen, as we proceed, that some of these coats are found by microscopic 

 examination really to consist of two or more strata differing from each 

 other in texture, and therefore reckoned as so many distinct coats by some 

 authorities. 



Internal coat. This may be raised from the inner surface of the arte- 

 ries as a fine transparent colourless membrane, elastic but very easily 

 broken, especially in the circular or transverse direction, so that it cannot 



