92 Comparative Anatomy } or an Atlcmpl at 



The quantity of pap always found in the sap-vessels plainly 

 shows their office : if then there were relurnivg sap-vessels, 

 would they not be equally plain and nisille, whether iti the hark 

 or wood? Besides, except the inner-lark vessels, all the cylin" 

 ders that can convey juices pass either round ihs tree or in a 

 transverse direction. 



I have but one more argument with which I shall trouble the 

 reader. It is said that the sap after mounting to the top of the 

 tree all descends again, and returns through the l>ark. Now the 

 bark is in summer one-tenth or eighth part of the wood, and 

 preserves in its decrease (as it mounts the tree) nearly the sanib 

 ratio. Suppose, therefore, a o to be the bark; and I' b the 

 wood ; and as the wood u as fiill of snp-vessels, it is not j>ossible 

 to cram them in less tiian /(0///A0/ space: the ratio l; c must, 

 therefore, be added to a a, yet not increase its size. This is 

 indeed adding much to a little and making it less. Had Nature 

 intended the sap-vessels or juices to return inio the hark, a con- 

 spicuous place would have l>een allotted for them; but they are 

 now sought in vain. It will be remembered that it is at all 

 times the sap is supposed to circulate when the bark is quite full 

 of its own juices, the sap must indeed then be invisible. 



The absorbent vessels are perhaps both in the animal and 

 vegetable one of the most curious parts of their formation. In 

 the human body they are various: those that open their extre- 

 mities under the different surfaces of the skin are called ex- 

 halants — of these the vegetables possess but a few, since they per- 

 spire not, and it is only under the first cuticle of the leaf they 

 are discovered : but there are oihcr absorbent vessels which take 

 in from the atmosphere (even in the human frame) many juices 

 and gases favourable to health, and which I doubt not increase 

 the nourishment of the body ; and though they are not nciir so 

 mimeroits as the hairs of vegetables (which are of such a variety 

 of kinds), yet they so increase with the vigour of the body, and 

 in hard-working people, that I doubt not they are intended to 

 supply much nutriment. In the vegetable it is well known that 

 they do so, and that they have absorbent vessels leading to each 

 hair, to throw the matter thus acquired ijito the nourishing ves- 

 sels ; and probably they v,ould not increase under such existing 

 circumstances in the human body, but for some such purpose. I 

 doubt not but that those who eradicate hairs, from the silly no^ 

 tion of beautifying their Maker's work, acquire many disorders 

 in the skin that would never be theirs but for this foolish cus- 

 tom. All hairs arise from two distinct capsules, one within the 

 other, having a certain degree of moisture between them ; and 

 the only difference between the human and common vegetable 

 hair is, that the latter has many more valves, and has frequently 



a spiral 



