140 PRACTICAL PHYSIOLOGY 



and provoke increased inspiratory movements. To prevent this the 

 vagi may be frozen instead of being cut. When both vagi are divided 

 note that the respiration is both slowed and deepened. The respiratory 

 exchange of gases, however, is not altered. Positive and negative 

 ventilation no longer produce an effect. When the central end of one 

 vagus is tetanised there results inspiratory spasm; with a very weak 

 current expiratory spasm may result. Excitation of the central end of 



FIG. 123. Expiration spasm of the diaphragm produced by weak stimulation of the 

 vagus. The down stroke represents inspiration, the up stroke expiration. The signal 

 line shows the duration of stimulation. (Fredericq and Nuel.) 



the superior laryngeal nerve provokes expiratory spasm. Division of 

 the phrenic nerves paralyses the diaphragm. If the tracheal tube be 

 connected with a water manometer or tambour immediately after the 

 death of an animal, and the peripheral end of the vagus be excited, a 

 rise of pressure will be observed, due to the constriction of the bronchial 



A 



D 



a/ 



FIG. 124. Inspiratory spasm of the diaphragm produced by excitation of the vagus 

 during the period shown by the signal a, 6. The down stroke represents inspiration, 

 the up stroke expiration. (Fredericq and Nuel.) 



muscles. The effects of positive and negative ventilation, and of vagus 

 excitation and section, may be observed by merely inspecting the 

 respiration of the animal. 



The Respiratory and Vasomotor Centres. The arterial pressure is 

 recorded, and the occipito-allantal membrane is exposed and opened. 

 The upper end of the cervical cord is then divided. After the initial 

 convulsion due to the lesion the respiratory movements of the thorax 

 cease, but those of the alae nasi and mouth continue for a minute or two. 

 The arterial pressure falls. The animal dies of respiratory paralysis. 

 If artificial respiration be established the circulation will continue, but 

 the arterial pressure will be low. The pressure will be raised very 

 greatly by tetanisation of the lower end of the divided cord. 



By observing the effect of division of the mid-brain in another animal 

 the respiratory centre and vasomotor centres can be localised to the 

 spinal bulb. 



