PHYSIOLOGICAL ANATOMY OF THE ARTERIES. 65 



scopic size. The branches, with the exception of the intercostal arteries, which make 

 nearly a right angle with the thoracic aorta, are given off at an acute angle. As a rule, 

 the arteries are nearly straight, taking the shortest course to the parts which they sup- 

 ply with blood; and, while the branches progressively diminish in size, but few are 

 given off between the great trunk and the small vessels which empty into the capil- 

 lary system. Haller counted but twenty branches of the mesenteric artery between the 

 aorta and the capillaries of the intestines. So long as a vessel gives off no branches, its 

 caliber does not progressively diminish ; as the common carotids, which are as large at 

 their bifurcation as they are at their origin. There are one or two instances in which 

 vessels, although giving off numerous branches in their course, do not diminish in size for 

 some distance ; as the aorta, which is as large at the point of division into the iliacs as 

 it is in the chest, and the vertebral arteries, which do not diminish in caliber until they 

 enter the foramen magnum. With these exceptions, as we recede from the heart, the 

 caliber of the vessels progressively diminishes. It has long been remarked that the 

 combined caliber of the branches of an arterial trunk is much greater than that of the 

 main vessel ; so that the arterial system, as it branches, increases in capacity. 



The arrangement of the arteries is such that the requisite supply of blood is sent to 

 all parts of the economy by the shortest course and with the least expenditure of force 

 from the heart. Generally, the vessels are so situated as not to be exposed to pressure 

 and consequent interruption of the current of blood ; but, in certain situations, as about 

 some of the joints, there is necessarily some liability to occasional compression. In 

 certain situations, also, as in the vessels going to the brain, particularly in some of the in- 

 ferior animals, it is necessary to moderate the force of the blood-current, on account of 

 the delicate structure of the organs in which they are distributed. Here Nature makes 

 a provision in the shape of anastomoses, by which, on the one hand, compression of a 

 vessel simply diverts, and does not arrest the current of blood, and, on the other hand, 

 the current is rendered more equable and the force of the heart is moderated. 



The arteries are provided with membranous sheaths, of greater or less strength, as 

 the vessels are situated in parts more or less exposed to disturbing influences or acci- 

 dents. 



Researches into the minute anatomy of the arteries have shown that they are pos- 

 sessed of three pretty well marked coats. As these vary very considerably in arteries of 

 different sizes, it will be convenient, in their description, to divide the vessels into three 

 classes : 



1. The largest arteries ; in which are included all that are larger than the carotids 

 and common iliacs. 



2. The arteries of medium size ; that is, between the carotids and iliacs and the 

 smallest. 



3. The smallest arteries ; or those less than ^ or -fa of an inch in diameter. 



The largest arteries are endowed with great strength and elasticity. Their external 

 coat is composed of white, or inelastic fibrous tissue, with a few longitudinal and oblique 

 fasciculi of involuntary muscular fibres. This coat is no thicker in the largest vessels 

 than in some of the vessels of medium size ; and in some medium-sized vessels it is 

 actually thicker than in the aorta. This is the only coat which is vascular. 



The middle coat, on which the thickness of the walls of the vessel depends, is com- 

 posed chiefly of the yellow elastic tissue. This tissue is disposed in numerous layers. 

 First we have a thin layer of ramifying elastic fibres, and then a number of layers of 

 elastic membrane, with numerous oval longitudinal openings, which has given it the 

 name of the " fenestrated membrane." Between the different layers of this membrane 

 are found a few unstriped or involuntary muscular fibres. These muscular fibres, how- 

 ever, are not numerous and have but little physiological importance. A small portion 

 of the aorta and pulmon ary^artery next the heart is entirely free from muscular fibres. 

 In the largest arteries, the/fibres are arranged in fasciculi, with amorphous and fibrous 

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