Lesson xxxvm.] CONCOCTION. 291 



descending aorta) whose strong pulsation presses 

 its yielding sides, and compels the chyle and lymph 

 to mount in an upright ascent. Fourthly, it must 

 be observed, that this duct is accommodated with 

 valves, which permit its contents to move upwards 

 by every compression, hut never to fall back again. 

 Thus terminates the second stage of Concoc- 

 tion, when the chyle falls into the heart. And it 

 may be seen that in the progress through these 

 two stages, our aliment has been mixed accurately 

 with all the nourishing juices and with all the sub- 

 stances or principles which compose the blood, 

 viz. saliva, mucus, lymph, bile, water, salts, oil, 

 and spirits. 



But here it will be proper to take notice, that 

 .he most fluid and subtile parts of our aliment, 

 before and after it is elaborated into chyle, pass 

 into the blood by certain absorbent veins dispersed 

 all over the mouth, gullet, stomach, and intestines: 

 when we consider how quickly refreshment and 

 strength are communicated to weary, faint, and 

 hungry people, immediately upon drinking a glass 

 of good wine, or eating any cordial spoon-meat, 

 this remark will appear ihe more obvious. 



The thiid stage begins where ihe chyle mingles 

 with the blood, and, falling soon in;o the right ven- 

 tricle of the heart, is from thence propelled into 

 the lungs. It will appear that the lungs are the 

 principal instruments of converting the chyle into 

 the blood ; especially if we consider their struc- 

 ture, first with regard to the air-vessels of which 

 they are composed, and secondly with regard to 

 o 3 their 



