AND PHYSIOLOGY. 



223 



be imperfectly supplied. And apoplexies and congestions 

 would be much more frequent than they now are in the vital 

 organs, if the arteries did not branch off at nearly a right 

 angle. 



386. Why a large Amount of Arteries and Veins — 

 The necessity for such a large amount of arteries and veins, 

 and lhair numerous connections with each other, is evident 

 from the great variety and extent of the tissues, and also 

 from the great liability to obstruction from inflammations, ac- 

 cidents, or even the ordinary compression of clothing. But 

 arranged as these vessels are in the body, it is a very difficult 

 thing by any mechanical means entirely to check the flow of 

 blood to any part. But were it not for this system of ample 

 inosculation the amputation of a portion of the body would 

 generally produce fatal results. 



387. Comparative Capacity of the Arteries and Veins. 

 — If all the arteries in the body were to be made into a single 

 vessel, the capacity of it would be much less than that of a 

 similar vessel made by the union of all the veins. This dif- 

 ference in capacity will in part result from the feebler char- 

 acter of the forces which propel the blood through the veins 

 than through the arteries, and consequently venous obstruc- 

 tion could not be so easily overcome as if it were in the ar- 

 teries ; and in order to compensate for deficiency in power, an 

 increased amount of tubing is provided. 



388. Forces of the Arterial Circulation. — Theinquiry 

 naturally arises, What are the forces that send the blood 

 through the circulatory channels ? 



389. Contraction of the Hear t — Elasticity of the Mid- 

 dle Coat. — In the first place the contraction of the heart is 

 the most essential force in driving the blood outwards. This 

 force has been estimated at thirteen pounds, though the re- 



Why do the arteries near the heart branch off at a large angle ? 3S6. Why is there so 

 large an amount of arteries and veins ? 387. What is the comparative capacity of the 

 veins and the arteries ? 3S8. What are the forces of the circulation ? 889. What is said 

 of the contraction of the heart ? What is the estimate of the force that it exei ks 2 



10* 



