PRACTICAL EXERCISES 289 



or A.C.E. mixture. Insert a cannula into the trachea (p. 186), and 

 connect it with the large bottle by a tube. Connect the bottle 

 with a recording tambour adjusted to write on a drum, and regulate 

 the amount of the excursion of the lever by slackening or tightening 

 the screw-clamp. Set the drum off at slow speed, and take a 

 tracing. 



(b) Then disconnect the cannula from its tube. Dissect out the 

 vagus in the lower part of the neck, pass a ligature under it, but do 

 not tie it. Connect the cannula again with the bottle, and while a 

 tracing is being taken ligature the vagus. Cut below the ligature 

 and stimulate its central end with weak shocks, marking the time 

 of stimulation on the drum. Repeat the stimulation with strong 

 shocks, and observe the results. 



(c) Apply a strong solution of potassium chloride with a camel's- 

 hair brush to the central end of the vagus while a tracing is being 

 taken, and observe the effect. 



(d) Isolate the sciatic nerve (p. 198), ligature it, and cut below the 

 ligature. Stimulate its central end while a tracing is being taken. 

 The respiratory movements will be increased. 



(e) Disconnect the cannula, and isolate the vagus on the other 

 side. While a tracing is being taken, divide it. The respiratory 

 movements will probably at once become deeper and less frequent. 



(/) Again disconnect the cannula. 

 Isolate the superior laryngeal branch 

 of the vagus. This will be found 

 entering the larynx at the point 

 where the laryngeal horn of the 

 hyoid bone is connected with the 

 thyroid cartilage. If the finger *is 

 passed back along the upper border 

 of the thyroid cartilage, this point , 1% 



will easily be felt. Ligature the 

 nerve, and divide it between the FIG. 130. STETHOGRAPH (CRILE). 

 larynx and the ligature. Recon- 

 nect the cannula. Take a tracing first with weak, and then with 

 strong stimulation of the central' end of the superior laryngeal. 



(g) Make an incision through the abdominal wall in the linea alba, 

 and study the movements of the diaphragm. Find the nerves from 

 which the phrenics take origin in the neck. In the dog they arise 

 from the fifth, sixth, and seventh cervical nerves. Divide the 

 phrenic fibres on one side, and observe that the diaphragm on the 

 corresponding side is now paralyzed. 



(h) Insert a cannula into the carotid artery. While a respiratory 

 tracing is being taken, allow blood to flow from the artery. Dyspnoea 

 and exaggeration of the respiratory movements will be seen when a 

 considerable quantity of blood has been lost. Mark and varnish the 

 tracings. 



In the whole of this experiment the tracheal cannula is to bs dis- 

 connected, except when the lever is actually writing on the drum, in 

 order that the period during which the animal must breaths into the 

 confined space of the bottle may be diminished as much as possible. 

 Instead of the method described, the stethograph shown in Fig. 130 

 may be used to obtain respiratory tracings from animals, a broad 

 canvas band being put round the animal's chest. To each end of 

 this band is clamped with sufficient tension a strong thread (F), 

 fastened to a small metal disc on the inside of the rubber dam closing 



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