160 PRACTICAL PHYSIOLOGY 



Note the effect of loosening the ligature. Now tie a ligature on 

 the pulmonary artery ; the right side of the heart becomes engorged 

 and the left empty. Loosen the ligature and observe the effects. 

 The heart is then excised and the pulsations studied. 



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

 the brain of an 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 prevents the over-dis- 

 tension 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), and observe that 

 the heart fills (a) on compressing the abdomen from below upwards, 

 (6) on sinking the animal in a bath of water up to the level of the 

 heart. In (6) the weight of the water outside tends to balance 

 the weight of the blood within. 



The vagus nerve may be found at the side of the neck, and the 

 effect of its excitation noted. Reflex inhibition of the heart is 

 very easily brought about by striking the abdomen or gills, or 

 pinching the tail of the eel. 



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

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

 artificial respiration established at once. 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 sym- 

 pathetic exercises a tonic action. On exciting the peripheral end 

 of the nerve with the faradic current, the vessels in the ear will 

 be seen to constrict, and this will take place to such a degree that 

 all the smaller vessels will disappear from view. The ear will at 

 the same time become several degrees cooler. Note that the latent 

 time is considerable between the excitation and the effect. Note 

 that the pupil also dilates, the nictitating membrane retracts, and 

 the palpebral tissue is widened. The eyeball at the same time 

 projects forwards. The pupillo-dilator fibres arise from the first 

 three thoracic anterior roots, the vaso-constrictor fibres from the 

 second to the fifth, and even to the seventh, in the rabbit. If the 

 superior cervical sympathetic ganglion be painted with nicotine, 

 excitation of the preganglionic fibres will no longer have any effect 



