GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 377 



becomes more or less venous (Sanderson). 1 In dogs the simultaneous divi- 

 sion of the two vagi usually reduces the number of the respiratory move- 

 ments by one-half (Reid). In pigeons the number falls to one-eighth, whilst 

 the depth of each inspiration is only increased as 1 : 2.5, the respiratory 

 activity therefore being diminished to about one-third of its original amount 

 (Rosen thai)." In some cases, after division of the Vagi, the respiration, 

 alters in character, the inspiration becoming sudden and spasmodic, with 

 long intervening pauses, but usually the difference is such as is expressed in 

 the following tracing given by Dr. Sanderson, in which the movements are 



FIG. 163. 



o. Tracing of the respiratory movements of the cat. a before, 6 after division of both vagi. 



seen to be more slowly performed, but much deeper and fuller. The influ- 

 ence of the Vagi upon respiration may also be shown by the application of 

 an electrical stimulus of moderate intensity to the centric extremity of the 

 divided nerves of one side. The effect will be observed to vary according to 

 whether the animal is breathing naturally, or is in a state of apnoea or dysp- 

 ncea. If it be breathing naturally, or be in a state of eupuoea, the imme- 

 diate effect of slight irritation is an acceleration of the respiratory move- 

 ments, whilst if the stimulus be stronger temporary arrest of respiration in 

 inspiration occurs, owing to contraction of the diaphragm without the par- 

 ticipation of the other respiratory muscles. If the excitation be still stronger, 

 the respiratory movements are completely suspended, the diaphragm passing 

 into a tetanic condition, which lasts as long as the stimulus is applied. In 

 apncea, when the blood is surcharged with oxygen, the effect is negative; in 

 dyspmea, when the blood is deficient in oxygen, all the accessory muscles 

 are brought into action, and the chest remains for a time in a state of tetanic 

 expansion. 3 When the animal is depressed by chloral, and its nerves are 

 exhausted, stimulation of the vagi, instead of being followed by contraction, 

 is followed by relaxation of the diaphragm, which is regarded as the effect 

 of the stimulation of the inhibitory fibres probably contained in the recur- 

 rents. 4 ( 297, 301, 302.) 



297. The branches of the vagi supplying the larynx possess special attri- 

 butes, which are comparable with those of the inhibitory nerves of the heart, 

 for if moderate excitation be applied either to the superior or to the inferior 

 laryngeal nerve the iuspiratory movements are diminished in extent and 

 frequency, whilst if the excitation be more energetic, arrest of respiration 

 with strong contraction of the expiratory muscles is produced (Breuer) 5 . 

 The superior laryngeal nerves are more sensitive to irritation than the in- 

 ferior. The experiments of various observers have afforded evidence that 



1 See Sanderson's Chapter on Respiration in the Handbook to the Physiological 

 Laboratory. 1873. 



2 Rosenthal, Comptes Itendns, 1861, p. 754. 3 Burdon-Sanderson, Op. cit., p. 311. 

 4 See Burkart, Pfliiger's Archiv, Band i, 1868, p. 107. 



6 Sitz.-ber. d. Wien. Akad , 1868, No. xii. 



25 



