2i6 A MANUAL OF PHYSIOLOGY 



with the common carotid. The respiratory movements imme- 

 diately stopped, the pulse was slowed, and death occurred 

 in thirty minutes (Rouse). The superior laryngeal fibres, 

 unlike those of the vagus proper, do not appear to be con- 

 stantly 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 respiration. They can be easily excited in the intact 

 body by thermal and mechanical stimulation. A cold 

 bath, for instance, usually causes acceleration and deepen- 

 ing 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. 



Another set of afferent nerves that seem to have 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 muscular 

 exercise. This seems to be brought about in part through 

 the stimulation of those afferent muscular nerves either by 

 mechanical compression of their terminal ' spindles,' or by 

 the chemical action on them of certain waste products 

 produced in contraction. But this cannot be the only way 

 in which the respiratory centre is affected by muscular 

 activity. For everybody is agreed that 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 connected 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 excitation of afferent 

 respiratory nerves whose connection with it is still intact 



