328 DETAILED SYNTHESIS. 



939. ARTERIES are very firm tubes that commence 

 by a single trunk from each heart, and lead, in one case, 

 into the lungs, and, in the other, into all parts of the 

 body. 



940. THE NUMBER AND CAPACITY or THE ARTERIES 

 is only about one third that of the veins in the system 

 generally, and about one half of that in the lungs ; but 

 as the arteries receive the blood directly from the heart, 

 it flows rapidly through them. 



941. THE ARTERIES OF THE SYSTEM ARISE (Fig. 44) 

 from the left, or systemic heart, by a single trunk (Aorta) 

 that turns down with a beautiful arch, from which 

 branches sweep out into the arms and lead directly up 

 to the head. The main subdivisions above and below 

 are noticeably few and large, throughout their length 

 giving off very numerous small branches and twigs. It 

 is also noticeable that all the branches lead off at acute 

 angles to the current of the Blood. The large ones take 

 a deep course in the limbs along the inner side of the 

 bones, always winding into the flexures of the joints ; 

 all of which facts, and more, are admirably illustrated 

 by Figs. 214, 215. 



942. THE ARTERIES ARE CONSTRUCTED of an inner 

 coat like that of the heart and tfe veins, and an outer 

 coat (sometimes subdivided) of sinewy fibres, the inner 

 part mostly of the elastic variety, together with some 

 unstriated muscular fibrillae. The outer coat is much 

 thicker than that of the veins ; these are easily com- 

 pressed, while the arteries maintain a cylindrical form, 

 unless forcibly compressed. 



943. THE ELASTIC AND MUSCULAR STRUCTURE OF THE 

 ARTERIES allows them to distend when the Heart forces 

 the Blood into them, and, when its action intermits, that 

 of the arteries, like that of the compressed air in fire-en- 

 gines, continues to propel the Blood onward. 



939. What are -? 940. What is -? 941. How do ? Describe Fig. 44. 

 942. How are ? 948. What the nse of - ? 



