370 MANIPULATION. 



upon this after a few trials, especially if the glass has been 

 previously wetted. A frog so mounted is capable of exhibit- 

 ing many of the eifects of inflammation ; if, for instance, a 

 spot in the web be touched with the point of a needle, or a 

 small drop of alcohol or other stimulating fluid be placed upon 

 it, the circulation will stop in that part for a longer or shorter 

 period, according to the amount of injury inflicted, the vessels 

 in the neighbourhood will soon become turgid, and even 

 sometimes be entirely clogged up with blood ; if no further 

 stimulus be applied, they will be seen to rid themselves of 

 their contents as easily as they became fuU, and, after a time, 

 the circulation will be restored in every part. For those who 

 are unacquainted with the parts which may be observed by 

 the microscope in the foot of the frog, it may be as well here 

 to state that the majority of vessels in which the blood is seen 

 to circulate are veins and capillaries; the former may be 

 known by their large size and by the blood moving in them 

 from the free edge of the web towards the leg, also by their 

 increase in diameter in the direction of the current ; the latter 

 are much smaller than the veins, and their size is nearly 

 uniform, the blood also circulates in them more quickly ; the 

 arteries are known by their small size, and by the great 

 rapidity with which the blood flows in them, they are far 

 less numerous than either of the other vessels, and, generally 

 speaking, only one can be recognised in the field of view at a 

 time ; in consequence of their being imbedded deeper in the 

 tissues of the web than the other vessels, the circulation 

 cannot be so well defined as in the latter. The black spots of 

 peculiar shapes that occur in all parts of the web are cells of 

 pigment, and the delicate hexagonal nucleated layer, which, 

 with a power of one hundred diameters, can be seen investing 

 the upper surface of the web, is tesselated epithelium. 



Method of Viewing the Circulation in the Tongue of the Frog. — 

 The organ which, on account of the complexity of its structure, 

 is the best adapted for examining the circulation of the blood, 

 is the tongue of the frog, for into this enter nearly all the 

 anatomical elements, viz., arteries, veins, capillaries, muscles, 

 nerves, glands, membranes, &c., representing, in fact, almost 



