GENERAL ANATOMY OF THE SANGUIFEROUS SYSTEM. 213 



fibres : for if it had been dependent on elasticity, it must have 

 reappeared when the distending power was withdrawn. 



It therefore seems certain, that the arteries have a power of 

 contraction different from that which depends upon elasticity ; 

 but whether this depend upon muscular fibres superadded to 

 them, or upon an irritable quality in the ordinary elastic fibres 

 of blood-vessels, is a question which is not perhaps completely 

 decided. 



Fig. 173.* — The middle coat of the 



arteries is composed of flat 

 fibres and bundles of fibres, 

 which surround the vessel in 

 a circular direction. They 

 are now generally believed 

 to be not muscular in their 

 structure, and do not exist 

 in veins. In the ox and the 

 elephant, they are particu- 

 larly well developed, in both 

 of which I have examined 

 them with attention, but 

 without discovering any evidence of muscularity in their com- 

 position. Berzelius has shown that the fibres of the elastic 

 coats of the arteries differ chemically from those of the muscles. 

 Muscular substance is soft and lax, and contains nearly three- 

 fourths iis weight of water, and has the same chemical proper- 

 ties as tlie fibrine of the blood. The substance of the arterial 

 coat is dry and contains no fibrine, and Dr. Hodgkin has 

 observed that its fibres do not present the transverse striae seen 

 on the muscular fibre. The arteries are eminently elastic, and 

 to this they owe their property of contracticfh, when they have 

 been distended, beyond their usual dimension. It is by virtue 

 of this elasticity also, that the arteries retain their tubular form 

 after death, when they are emptied of blood. The reaction of 



* Elastic tissue from the outer part of tlie middle fibrous coat of the aorta, 

 as it appears when magnified 300 diameters. The intertangled fibres and 

 elongated cells common to the elastic tissue are well shown. 



