RESPIRATION 229 



However this may be, the facts we have been discussing have 

 an importance of their own, apart from any hypothetical ex- 

 planations of them. Some of them have been more than once 

 unintentionally illustrated on man. In one case the left vagus 

 trunk was included in a ligature with the common carotid. The 

 respiratory movements immediately stopped, the pulse was 

 slowed, and death occurred in thirty minutes (Rouse). The 

 superior laryngeal fibres, unlike those of the vagus proper, are 

 not constantly in action, as section of both nerves has no effect 

 on respiration. Any source of irritation in the larynx may 

 stimulate these fibres and produce a cough, which may also be 

 caused by irritation of the pulmonary fibres of the vagus. 



The cutaneous nerves, and especially those of the face (fifth 

 nerve), abdomen and chest, have a marked influence on re- 

 spiration. They can be easily excited in the intact body by 

 thermal and mechanical stimulation. A cold bath, for instance, 

 usually causes acceleration and deepening of the respiratory 

 movements ; and the efficacy of mechanical stimulation of 

 sensory nerves in stirring up a sluggish respiratory centre is 

 well known to midwives, who sometimes slap the buttocks of 

 a newborn child to start its breathing. The reflex expiratory 

 standstill caused in rabbits by inhalation of such sharp-smelling 

 substances as ammonia, acetic acid, and tobacco-smoke is due 

 to afferent impulses passing up the trigeminus fibres from the 

 mucous membrane of the nose, and is still obtained'after section 

 of the olfactory nerves. 



Another set of afferent nerves which have been supposed by 

 some to bear an important relation to the respiratory centre 

 are those which supply the muscles. We have already noticed 

 that the frequency of respiration is greatly augmented by mus- 

 cular exercise. The simplest explanation would seem to be that 

 afferent muscular nerves are stimulated either by mechanical 

 compression of their terminal ' spindles,' or by the chemical 

 action on them of certain waste products produced in contrac- 

 tion. It is quite likely that this is one way in which the adjust- 

 ment is achieved. But this is not the only, and perhaps not 

 the most important, way. For an increase in the respiratory 

 movements is caused by tetanizing the muscles of a limb whose 

 nerves have been completely severed, and which is indeed con- 

 nected with the rest of the body by no other structures than its 

 bloodvessels. This can only be due to two things : a direct action 

 on the respiratory centre by the blood that has passed through, 

 and been altered in, the contracting muscles, or an action 

 exerted by the blood indirectly on the centre through the excita- 

 tion of afferent respiratory nerves whose connection with it is still 

 intact for example, the other muscular nerves or the pul- 



