ELEMENTARY DEMONSTRATIONS 



125 



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

 brain of a grass snake or eel. 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 pre- 

 vents 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 n 

 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), and observe that the heart fills (a) on compres- 

 sing 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 excita- 



7 1'iG. 102. Artificial 



tion noted. In the eel reflex inhibition of the heart schema. 



is very easily brought about by striking the abdomen or gills. 





CHAPTER XXXIII. 

 VASO-MOTOR SYSTEM. CIRCULATION TIME. 



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

 one with a white ear; the animal is anaesthetised with chloral 

 or urethane; the ear is shaven and fixed by threads to a loop 

 of stout wire. This wire is clamped in front of the lantern, so that the 

 blood vessels in the ear can be plainly seen. The cervical sympathetic 

 is exposed in the neck, where it lies behind the carotid artery, and is 

 traced up to the superior cervical sympathetic ganglion. The thread is 

 tied round the nerve, and the latter is cut. Observe that at this 

 moment the blood vessels in the ear dilate and the ear becomes warmer. 

 The palpebral fissure at the same time becomes narrowed. The change 

 will be much more marked had the ear of the rabbit been previously 

 exposed to cold. The cervical sympathetic exercises a tonic action. 



