CIRCULATION [Lesson xxxix. 



sidered the cavities of the heart, by means of 

 which the Blood is propelled through the body. 

 To these I now proceed. 



The arteries are blood-vessels consisting of a 

 close texture of strong elastic fibres, woven in 

 rarious webs, laid in different directions, and inter- 

 spersed with an infinity of delicate nerves, veins, 

 and minuter arteries. They are divided and sub- 

 divi^jd into numberless branches and ramifica- 

 tions, that become smaller and smaller as they 

 recede from the heart, until at last their extremi- 

 ties are found much more slender than the hairs 

 of our heads, (and are therefore called capillary 

 arteries), which either unite in continued pipes 

 with the beginnings of the veins, or terminate in 

 mall receptacles, from which the veins derive 

 their origin. The arteries have ,no valves, but 

 only have their trunks spring from the heart : they 

 throb and beat perpetually whilst life remains j 

 their extremities differing in the thickness of their 

 coats and some other particulars, according to the 

 nature of the part which they pervade. All the 

 arteries in the lungs (except the small ones that 

 convey nourishment to them) are derived from 

 the great pulmonary artery, which issues from the 

 right ventricle of the heart. And all the arteries 

 in the rest of the body proceed from the aorta, 

 (which obtained this name, because the ancients 

 thought it contained air only), whose trunk springs 

 from the left ventricle of the heart. 



The veins resemble the arteries in their figure 

 and distribution $ but their cavities are larger, and 



thier 



