RESPIRATION. 19 T 



thing through the mouth. The action, indeed, of the tongue and floor 

 of the mouth in sucking may be compared to that of the piston in a 

 syringe, and the muscles which pull down the os hyoides and tongue, to 

 the power which draws the handle. 



Influence of the Nervous System in Respiration. 



Like all other functions of the body, the discharge of which is neces- 

 sary to life, respiration is essentially an involuntary act. Unless this 

 were the case, life would be in constant danger, and would cease on the 

 loss of consciousness for a few moments, as in sleep. It is, however, 

 also necessary that respiration should be to some extent under the con- 

 trol of the will. For were it not so, it would be impossible to perform 

 those voluntary respiratory acts which have been just discussed, such as 

 speaking, singing, and the like. 



The respiratory movements and their rhythm, so far as they are in- 

 voluntary and independent of consciousness, as they are on all ordinary 

 occasions, are under the governance of a nerve-centre in the medulla 

 oUongata which corresponds in position with the origin of the pneumo- 

 gastric nerves ; that is to say, the muscles concerned in the respiratory 

 movements, are excited by stimuli which issue from this part of the 

 nervous system, and which are conveyed by the various motor nerves 

 supplying the muscles. These nerves are the phrenics and intercostals 

 chiefly. On division of one phrenic, for example, the corresponding 

 half of the diaphragm supplied by it ceases to take part in the respira- 

 tory movement, and on division of both nerves, the whole muscle ceases 

 to act. Similarly, division of the intercostal nerves one by one produces 

 cessation of action of the muscles supplied by them. To what extent 

 the medullary centre acts automatically, i. e.. how far the stimulus orig- 

 inates in it, or how far it is merely a nerve-centre for reflex action, is 

 not certainly known. 



It is clear, however, that the medullary centre is bilateral or double, 

 since the respiratory movements continue after the medulla at this point 

 is bisected in the middle line. 



There is considerable evidence in favor of its automatic action. Thus 

 it has been shown that if the spinal cord be divided below the medulla, 

 so that no afferent impulses can reach the centre from below, that the 

 nasal and laryngeal respiration continues. The only possible course of 

 the afferent impulses would, under such circumstances, be through the 

 cranial nerves; and when the cord and medulla are intact the division 

 of these nerves produces no effect upon respiration, and indicates that 

 they are not used for the transmission of afferent impulses to the medul- 

 lary centre. It appears evident, therefore, that afferent stimuli are not 

 absolutely necessary for maintaining the respiratory movements. The 



