in their Structure and Functions, 5f\^ 



tadpoles into water contained in a reservoir, from which the 

 light was excluded. The tadpoles grew, but did not become 

 froffs. He afterwards removed some of them into a situation 

 to which the light had free access. These soon lost their 

 tail and fins, and were converted into perfect frogs : but the 

 rest, which were kept in the dark, still continued in the state 

 of tadpoles. 



I have now endeavoured to explain the functions of ab- 

 sorption, of circulation, and respiration ; the manner by which 

 the chyle is taken up, and carried into the system ; how it is 

 converted in the lungs into blood ; and how the blood is dis- 

 tributed to every part of the body. I have likewise pointed out 

 how nearly the functions by which all these effects are pro- 

 duced in the animal frame, resemble those functions in the 

 vegetable system by which the crude sap is absorbed from 

 the earth ; how it is changed in the leaves into cambium ; and 

 how the cambium is conveyed all over the plant. In the 

 early part of my essay I remarked that every organised 

 being is composed of solid and fluid parts. The solids consist 

 of muscular, membranous, and nervous substance ; the fluids 

 consist of aqueous and other matter. In the animal frame, all 

 the solid and fluid substances are produced from the blood ; 

 in plants, they are produced from the sap. Skin, fat, brain, 

 muscle, membrane, saliva, tears, bile, and urine, are all eli- 

 minated from the blood; bark, wood, pith, pollen, oil, sugar, 

 &c., are all eliminated from the sap. The process by which 

 muscle is extracted from the blood, and that by which wood 

 is extracted from the sap, are termed secretion. There are 

 many substances existing in the blood, which, in consequence 

 of their peculiar chemical qualities, are enabled to escape 

 from the system by transuding through the sides of certain 

 organs ; this is considered as a species of secretion : but 

 secretion is properly the separation of substances from the 

 blood, which did not previously exist in this fluid. The or- 

 gans by which it is performed, in the animal frame, consist of 

 vesicles or hollow sacs. The glands, such as the liver, the 

 pancreas, &c., are composed of a congeries of these vesicles ; 

 among which bloodvessels and excretory ducts abundantly 

 ramify. The secreting organs of vegetables are precisely 

 similar to those of animals. They consist of hollow bodies ; 

 and these, when collected into clusters, constitute glands, which 

 are as abundant in the vegetable as in the animal system. 

 Various speculations have been offered with regard to the 

 theory of secretion. Van Helmont and Willis ascribed the 

 process to fermentation; Hunter, Blumenbach, Bichat, and 

 Abernethy ascribed it to the agency of the vital principle ; 



