ELEMENTARY EXPERIMENTAL PHYSIOLOGY 137 



proportional to the total cross section at that part. If the total cross 

 section of any one part of the circuit be dilated the velocity becomes 

 slower there, while it proportionately increases in the other parts. 

 This must be so if the blood continues to circulate round the whole 

 system in the same time. Vaso-dilatation in one part is normally 

 compensated for by constriction in other parts. 



Velocity in the Capillaries. Pith the cerebrum of a frog and 

 plug the hole. Lightly curarise the frog, and spread the web over 

 the hole in the web-board. Examine the circulation under the 

 microscope, and with the aid of an ocular micrometer and a clock 

 beating i seconds measure the time it takes for a red corpuscle to 

 move through ^ mm. Note in an arteriole that the red corpuscles 

 move the fastest in the axial stream, while the white corpuscles roll 

 slowly along the margin. 



Place on the web a drop of hot water (50-60 C.). The flow at 

 first is accelerated owing to vaso-dilatation, but soon slackens as 

 the red corpuscles clump together owing to the escape of the plasma 

 through the damaged capillary walls. 



The Influence of Gravity on the Circulation of the Snake. Pith 

 the brain of a grass snake or eel. (This experiment can be carried 

 out on the frog, but is less striking.) Fasten the animal on to a 

 board. Expose the heart, which may be seen beating beneath the 

 skin, about 2-3 inches below the mouth. Place the animal head 

 down in the vertical position. Notice the pericardium prevents the 

 over-distension of the heart by the weight of the super-incumbent 

 column of blood. Slit open the pericardium and observe the result. 

 The heart becomes greatly congested. This is especially marked in 

 the eel, when reflexly excited to writhe. Turn the animal head 

 uppermost. The heart gradually empties, and becomes at last pale 

 and bloodless. Slowly tilt the board and observe the blood as it 

 runs up the inferior vena cava and fills the heart Place the animal 

 again in the vertical posture (head up), arid observe that the heart 

 fills (a) on compressing the abdomen from below upwards (b) on 

 sinking the animal in a bath of water up to the level of the heart. 

 In (b) the weight of the water outside tends to balance the weight 

 of the blood within. 



The vagus nerve may easily be found at the side of the neck in 

 the snake, and the effect of its excitation noted. In the eel reflex 

 inhibition of the heart is very easily brought about by striking the 

 abdomen or gills. 



Demonstration of Vaso-Motor Nerves. A white rabbit is chosen, 

 or one with a white ear; the brain of the animal is pithed and 



