340 CIRCULATION. 



It is evident that, under certain circumstances, a larger 

 quantity of blood than usual may pass through these parts 

 without necessarily penetrating the true capillaries and thus 

 exerting a modifying influence upon nutrition. The changes 

 which are liable to occur in the quantity of blood, in the 

 force of the heart's action, etc., may thus take place without 

 disturbing the circulation in the capillaries, a provision which 

 the functions of the parts would seem to demand. 1 



Pulmonary Circulation. The vascular system of the 

 lungs merits the name, which is frequently applied to it, of 

 the lesser circulation. The right side of the heart acts simul- 

 taneously with the left, but is entirely distinct from it, and its 

 muscular walls are very much less powerful. The pulmo- 

 nary artery has thinner and more distensible coats than the 

 aorta, and distributes its blood to a single system of capil- 

 laries, which are located very near the heart. We have seen 

 that the orifice of the pulmonary artery is provided with 

 valves which prevent regurgitation into the ventricle. In 

 the substance of the lungs, the pulmonary artery is broken 

 up into capillaries, most of them just large enough to allow 

 the passage of the blood-corpuscles in a single row. These 

 vessels are provided with a single coat, and form a very close 

 network surrounding the air-cells. From the capillaries, the 

 blood is collected by the pulmonary veins, and conveyed to 



1 Before the publication of the researches of Suquet, Todd and Bowman men- 

 tioned the possibility of direct communications between the arteries and veins in 

 many parts of the body, and the probable existence of such communications in 

 some of the bones. 



" It is not improbable that further research may detect a direct communication 

 between arteries and veins, even in tissues, the greatest part of which is furnished 

 with a true capillary plexus. In the cancellated structure of bone, and the diploe 

 of the cranial bones, it seems highly probable that the arteries communicate im- 

 mediately with the veins at many points. Mr. Pag'et (Lectures on Inflammation} 

 describes a direct communication between the arteries and veins of the wing of 

 the bat, without any intermediate capillary plexus." TODD and BOWMAN, Physi- 

 ological Anatomy and Physiology of Man, American edition, Philadelphia, 1857, 

 p. 662. 



