HISTOLOGICAL DEMONSTRATIONS. 103 



1 66. The Prog's Web. Inject one, two, or three 

 minims according to size of frog of a watery solution of 

 curara, each minim of which contains 0*032 milligramme of 

 the poison. This dose is small, and will take from ten to 

 twenty minutes to produce paralysis. It is well, however, 

 not to give a larger dose, otherwise the vasomotor nerves 

 will be paralysed. 



When paralysis has supervened, tie a soft woollen thread 

 gently round the longest toe, close to its tip, and another 

 round the toe next to, and on one or the other side of it. 

 Lay the frog on its back, and stretch the web gently over a 

 triangular window in a piece of cardboard (A, Fig. 44) by 

 drawing the threads attached to the toes gently through the 

 slits a and a'. The web must be kept wet by the repeated 

 addition of water. 



(L.) Artery, vein, capillaries, and pigment cells. The 

 velocity of the stream in the artery is greater than in the 

 vein. This results from the calibre of the artery being less 

 than that of the vein, and from the greater pressure of the 

 blood in the former. 



(H.) (Tube of microscope shortened.) Examine the 

 manner in which the coloured and colourless corpuscles 

 move along the capillaries the former with comparative 

 rapidity, owing to their polished surfaces ; the latter tardily, 

 owing to an apparent tendency to adhesion between the 

 wall of the vessel and the corpuscle. In the arteries and 

 veins it may often be seen that the coloured corpuscles 

 move principally in the centre of the stream, while the 

 colourless corpuscles may often be found adhering to the 

 wall of the vessel, or moving tardily along it. The peri- 

 pheral part of the stream is often devoid of coloured 

 corpuscles (the " lymph space " of older observers). The 

 smaller velocity of the peripheral, as compared with the 

 central part of the stream, is owing to the friction between 

 the vessel and the blood, and the adhesion here and there 

 of the white corpuscles. 



167. The Frog's Mesentery. The student is not 

 permitted to do the following experiment, but it may be 

 shown to him by his teacher thus : The curarised frog is 



