898 ANGIOLOGY. 



into the system generally ; the veins returning it again to the 

 heart. Tlie circulation is, therefore, double — the Pulmonary or 

 lesser being performed by the right, and the Somatic or greater 

 circulation by the left side. Some writers speak of the portal 

 and chylo-poietic or visceral circulation as forming a third system, 

 but with respect to the heart it is not so, since they are merely 

 a branch of the somatic circulation. 



Artehies. 



These are tubes conveying the blood from the heart, a series 

 belonging to each circulation. The somatic and pulmonary 

 arteries leave the heart each by a single trunk, which subdivides 

 and terminates in capillaries, whence the venous radicles originate. 

 The volume of the secondary arteries exceeds that of the primi- 

 tive trunk, and similar relations exist, as far as the ultimate 

 divisions of the artery, their inclusive capacity increasing towards 

 their termination. Arterial branches arise at variable angles, 

 and the originating angle exerts a decided influence on the 

 velocity of the blood current. Blood passing from a primary 

 into a secondary vessel which arises from it at a right angle, as 

 in some branches from the aorta, must have its speed slackened ; 

 on the contrary, the velocity is little modified in vessels which 

 leave the parent trunk at a very acute angle, as do many of the 

 arteries of the limbs. 



The large arteries are usually deep-seated, occupying the cavi- 

 ties of the trunk and the inner surfaces of the limbs, where they 

 are less exposed to injury. When they pass over an articulation, 

 they are generally found on the flexor side. In general, the 

 direction of the larger arteries is straight, but in some parts they 

 are tortuous, as in the stomach, intestines, bladder, tongue, and 

 lips ; in fact, wherever the circulation might be impeded by 

 sudden functional change in the form or volume of the organ. 

 Sometimes this tortuosity is apparently for the purpose of moder- 

 ating the energy with which the fluid is propelled into delicate 

 structures, such as the brain and testicle ; small arteries which 

 run amongst loose structures generally become tortuous during 

 the contraction of the left ventricle. 



Arteries freely communicate with each other, forming what are 

 termed anastomoses ; by convergence, where two branches join at 

 an angle to constitute a single trunk, as the cerebro-spinal 



