vn] of the Seventeenth Century. 195 



" For other ducts, which serve to carry denser liquids, are not 

 " visible to the eyes until their several hair-like passages, after 

 " running a certain distance, join together to form a notable 

 " canal. What eye has ever been sharp enough to see the first 

 " beginnings of the lymphatic, or of the lacteal vessels, or even 

 " of the veins ? " He does not seem to have read Malpighi's 

 second tract, and Leeuenhoek had not yet made his more 

 convincing observations. " How much less are we likely to see 

 " these aereal ducts, which must be very short and exceedingly 

 " delicate. For these passages are not like the others, passages 

 " becoming joined together after running separately for some 

 " distance ; they only have to traverse the membranes of the 

 11 lungs, each following a very short and obscure path. For in 

 " order that the aereal particles should mix with the mass of 

 " blood in a state of fine division and in a most intimate 

 "manner, it is necessary that they should enter the blood 

 " through channels or rather orifices almost infinite in number, 

 " distributed here and there over the whole mass of the lungs. 

 " And indeed in lungs which have been prepared and dissected, 

 "holes, almost without number, like most minute points may 

 " be seen with the aid of the microscope. Whether however 

 " those points are the mouths of capillary air tubes or of vessels 

 " opening into the blood cannot be determined for certain. 



"Let us inquire in the next place what is that con- 

 stituent of the air which thus passes into the blood, which 

 " is so necessary for sustaining life that we cannot live for even 

 " a moment of time without it. And indeed it is very probable 

 " that certain particles of a nitro-saline nature, and those very 

 " subtle, nimble, and of very great fermentative power, are sepa- 

 " rated from the air by the aid of the lungs and introduced into 

 " the mass of the blood. And so necessary for life of every 

 " kind is that aereal salt (constituent) that not even plants can 

 " grow in earth the access of air to which is shut off. But if 

 " that same earth be exposed to air and so forthwith impreg- 

 " nated with that fecundating salt, it at once becomes fit again 

 " for growing. It is clear that even the very plants seem to 

 " have some need of breathing, some need of drawing air into 

 " themselves. 



13—2 



