I4O DEMONSTRATIONS. 



left ventricle produces a momentary increase of arterial pressure ; (2) that the 

 pressure increases after each inspiration, and sinks in the interval ; (3) that 

 during the rise of pressure, the pulsations are more frequent than during the 



fall. Excitation of the Cardiac end of the divided Vagus, 



by faradization, produces (if weak induction currents are used) diminution of 

 the frequency of the heart's pulsation and of the arterial pressure. If stronger 

 currents are used, the heart is arrested in diastole (see p. 85). 



[N.B. In each of the Demonstrations I., II., III., and IV., a rabbit is 

 used, which is rendered completely insensible by a suitable anaesthetic, and is 

 killed before recovery.] 



II. The Normal Respiratory Movements. Influence of the Vagus Nerve 

 and of its Centre. Apncea and Dyspnoea. 



The motions of a metal plate which is kept in constant contact with the 

 posterior surface of the central tendon of the diaphragm of the rabbit, by the 

 pressure of a spring are communicated by a long steel wire to the vertical arm 

 of a bell-crank lever. The horizontal arm of the lever is prolonged, and bears 

 a style by which an enlarged record of the respiratory motion of the diaphragm 

 is inscribed on the cylinder of the recording apparatus. The rate of movement 

 of the cylinder is the same as in the last demonstration. 



The inspiratory contraction of the diaphragm is expressed by the descent 

 of the writing style, its relaxation by the ascent, which is at first rapid, but 

 afterwards more gradual. 



Apnoea. When by excessive artificial respiration the circulating blood 

 becomes overcharged with oxygen, all respiratory movement ceases. On 

 discontinuing the injections of air, the respirations after a time begin again : 

 at first they are scarcely perceptible, but each exceeds its predecessor in 

 extent, until the normal is reached. 



Dyspnoea. When nitrogen containing an inadequate percentage of 

 oxygen is respired, the opposite effect to that described above is produced. 

 The respirations become more ample and more frequent, and the auxiliary 

 muscles are brought into action. No such effect is produced by an atmosphere 

 containing as much as ten per cent, of CO 2 , provided that the supply of 

 oxygen is sufficient. 



Excitation of the Superior Laryngeal Nerve. Excitation 



of the central end of the trunk of the superior laryngeal nerve, by faradization, 

 arrests the respiratory movements, the diaphragm becoming stationary in the 

 position of expiration. When extremely feeble currents are used, rhythmical 

 movements may continue at long intervals. Introduction of irritant gases or 

 vapours into the larynx produces similar effects. 



Similar excitation of the central end of the divided vagus, below the cricoid 

 cartilage, produces effects which differ according to the strength of the induction 

 currents employed. When currents of moderate strength are used, the 

 diaphragm remains during the excitation in the position of inspiration, the 

 state of contraction being, however, usually interrupted by momentary relaxa- 

 tions at short intervals. 



