PRACTICAL EXERCISES 301 



(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. 212), 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 



Fig. 135. Arrangement for Respiratory Tracing. Two glass tubes are inserted 

 through a cork in the mouth of the large bottle. One of them has a small piece of 

 indiarubber tubing on it, which is closed or opened, as may be required, by a 

 screw-clamp. The other is connected by a rubber tube with a recording tambour. 

 The tubulure at the bottom of the bottle is closed by a cork, through which 

 passes a glass tube, connected by a rubber tube with the tracheal cannula. If 

 no bottle with tubulure is available, it is only necessary to pass through the cork, 

 down to the bottom of the bottle, a third glass tube, which is connected with the 

 tracheal cannula. While a tracing is being taken the animal breathes the air 

 contained in the bottle. When this becomes vitiated the respiratory movements 

 are exaggerated and a normal tracing is no longer obtained. For this reason 

 the tracheal cannula must be connected with the bottle only at the moment 

 when a tracing is to be taken. The arrangement is most suitable for a small 

 animal. 



of the thyroid cartilage, this point will easily be felt. Ligature the 

 nerve, and divide it between the larynx and the ligature. Reconnect 

 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 



