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internal substance of the lungs is 14412 pounds. So 

 vast is the extent of the surface of the vesicles, on 

 which it was necessary the blood should be spread in 

 the finest capillary vessels, that each globule of blood 

 might, as it were, immediately receive the whole force 

 of the air, and thereby be broken into smaller parts, 

 lit for secretion and circulation. 



And hence we see the reason for the structure of 

 the lungs. 'For since all the blood is to pass through 

 them, in order to receive the effect of the air, and that 

 this could not be done unless it were diffused in very 

 small vessels ; it was necessary the surface on which 

 they were to be spread should be proportioned to their 

 number, and this is admirably well provided for by ttfe 

 fabric of the lungs. * 



If the diameter of the trachea at the time of every 

 expiration were the same in all, aud the weight of the 

 air always equal, the pressure on the lungs would be 

 always the same. But as the difference between its 

 least and greatest gravity is no less than a tenth part of 

 the whole, that pressure's likewise greater by a tenth 

 p:*rt at some times than it is at others. 



is a difference which the asthmatic must sent 

 sably feel, especially as they brea(he thicker, that is, 

 every expiration is performed in less time. In truth, 

 these feel a difference in the air, upon the greatest rise 

 and fall of the barometer, equal to above one third of 

 its pressure in ordinary breathing. 



The alternate dilatation and contraction of the 

 thorax are so necessary to animal life, that there is no 

 animal without this or something analogous to it.. Fishes 

 and insects have no dilatable thorax ; but fishes have 

 gills which receive and expel the water alternately, 

 whereby the blood-vessels suffer the same alterations of 

 dimension as those in our lungs do ; and insects have 

 air-vessels distributed through the whole trunk of .their 

 bodies. By these they communicate with the external 

 air through several vent-holes, to which are fastened 

 so many wind-pipes which send branches to all parts j 

 and seem to accompany the blood-vessels all over the 

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