74 ORGANISED FLUIDS. 



the mucous membrane of the tongue, to the thickness of 

 which they are entirely confined, as proved by the fact that, 

 when that membrane is dissected off, by means of a needle the 

 glands are raised along with it. Into each of these glands 

 may be seen entering it on one side, and quitting it usually 

 on the opposite, one of the smallest of the capillary vessels, 

 in which the blood corpuscle^ pass usually in single series, 

 this vessel in its passage through the gland describes usually 

 a tortuous course ; and within it the blood corpuscles are seen 

 to be in a state of increased and incessant activity, appearing 

 to move, as it were, in a vortex, this appearance resulting 

 from the curvatures described by the vessel. (See Plate VII. 

 figs. I, 2.) 



It might be expected that, in a gland of such simple con- 

 stitution, the exact process of secretion would be rendered 

 apparent; in this expectation, however, we are doomed to 

 disappointment, no action beyond that which we have already 

 related being visible within it. An endosmotic action does 

 doubtless take place between the contents of the gland and 

 those of the vessel which permeates it, whereby a peculiar 

 product is obtained from the blood, to be fashioned and 

 assimilated by certain powers inherent in the gland itself, 

 and the precise nature of which powers is unknown to us, and 

 it is probable that it never will be revealed. Pass we now 

 to the description of the circulation in the embryo of the 

 chick, which possesses points of interest distinct from those 

 observed in the tongue of the frog. 



CIRCULATION IN THE EMBKYO OF THE CHICK. 



The process by which the circulation in the embryo of the 

 chick is displayed is one which requires considerable delicacy 

 of manipulation ; the care, however, which it is necessary to 

 bestow upon it, for its successful exhibition, is amply repaid 

 by the surpassing beauty of the spectacle which presents 

 itself to the beholder. It is best seen in the third, fourth, 

 and fifth days of the incubation of the egg. 



