82 EXPERIMENTAL PHYSIOLOGY 



or a limb, may be placed in a plethysmograph, and the changes of volume 

 recorded. In the anaesthetised animal the following experiments can be made : 

 for many of them the Sherrington preparation can be used : 



1. Effect of exciting the afferent fibres of a peripheral limb-nerve. Expose 

 any limb nerve, tie a ligature tightly round the distal end of the part exposed. 

 Stimulate the central end. Observe the effects on arterial pressure, on heart 

 rate, and on respiration. 



2. Put a drop or two of amyl nitrite on cotton -wool and allow the animal to 

 inhale the vapour. Note the effect on blood-pressure. 



3. Effect of exciting the vagus. One of the vagi in the neck having been 

 isolated for a short distance, taking care to avoid injuring it, a thread is tied 

 tightly round it : this serves to hold it, and also severs the continuity of its 

 fibres, and is therefore equivalent to cutting it across, (a) Stimulate the end 

 below the ligature. Observe the effect on blood-pressure caused by cardiac 

 inhibition. (6) Stimulate the upper end. Observe the reflex effects on blood- 

 pressure and on respiration which are produced, (c) Stimulate the superior 

 laryngeal branch, which can be seen passing to the larynx. Observe the effect 

 on respiration and blood-pressure. 



4. Effect of cutting both vagi. The ligature of the one vagus which, as 

 just explained, severs its fibres produces little or no permanent effect on blood- 

 pressure or respiration. Now tie or cut the other vagus also. Notice the effect 

 (a) on respiration, (6) on blood-pressure. Repeat the excitation experiments. 



5. Inject solution of atropine sulphate (about 2 milligrams) into the jugular 

 vein. Notice the effect, if any, upon blood-pressure and respiration. Repeat 

 the excitation of lower and upper ends of vagus as in (1), and note results. 



6. If a rabbit is used, two other very fine nerves can be found in the neck 

 accompanying the carotid artery and vagus. One of these is the depressor : 

 it is a branch of the vagus or of the superior laryngeal. The other is the cervical 

 sympathetic : it passes above into the superior ganglion. Both these nerves 

 are to be tied low down. After tying the sympathetic, the pupil of the eye on 

 that side will be more contracted than the other, and the ear of that side will be 

 warmer and redder, (a) Stimulate the upper end of the sympathetic : the 

 pupil dilates, the third eyelid is retracted, some of the hairs on the side of the 

 head may be erected, and the arteries of the ear contract, so that the whole ear 

 becomes pale. (6) Stimulate the upper end of the depressor whilst the blood- 

 pressure is being recorded. After a long period of latency there is a fall of 

 pressure which lasts during stimulation of the nerve and for a short time after 

 its cessation. 



7. Inject into the jugular vein a few drops of an extract of the supra- 

 renal capsule of any animal. The extract is made by taking 20 c.c. of 

 Ringer's solution to each gram of suprarenal capsule, boiling and filtering. 

 Record the effect on blood-pressure. Notice the dilatation of the pupils, the 

 retraction of the third eyelid, and the pallor of the ears. If a plethysmo- 

 graph is being used, the record of the effect on the contained organ indicates 

 contraction of arteries. The experiment can be repeated more than once. 



8. Inject into the jugular vein a few drops of an extract of ox-pituitary. 

 The extract is made in the same way as the suprarenal extract. The posterior 

 lobe only should be used. Notice the effect of the extract upon the blood-pressure 

 and upon the plethysmograph record. If the injection is repeated after a short 

 interval, most of the results are not shown, or are much less marked. 



9. Kill the animal by asphyxia which may be effected by occluding the 

 trachea or by allowing carbon monoxide gas (or coal gas) to be respired. A con- 

 tinuous tracing showing the effects of asphyxia, both upon the respiratory 

 movements and upon the blood-pressure and heart-beats, may be recorded. 



The capillary circulation. The flow of blood in the smallest arteries and 

 veins and in the capillaries is observed with the microscope in transparent 

 parts of animals such as the web and mesentery of the frog, the tail of the 

 tadpole, and the mesentery of small mammals. 1 



1 For the methods of displaying these parts, see the author's Course of 

 Practical Histology, in which full details are given. 



