242 CIRCULATION. 



from the heart, the arteries branch, divide, and subdivide, 

 until they are reduced to microscopic 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 

 supply with blood ; and while the branches progressively di- 

 minish in size, but few are given off between the great trunk 

 and the minute vessels which empty into the capillary sys- 

 tem. Haller counted but twenty branches of the mesenteric 

 artery between the aorta and the capillaries of the intestines. 1 

 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, though giv- 

 ing 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. 2 "With these exceptions, as 

 we recede from the heart, the caliber of the vessels progres- 

 sively 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 cur- 

 rent of blood ; but in certain situations, as about some of the 

 joints, there is necessarily some liability to occasional com- 



1 Cyclopedia of Anatomy and Physiology, vol. i., p. 220 ; and HALLER, Ele* 

 menta Physiologies, tomus i., sec. i., 17. 

 * Ibid. 



