264 ESSENTIALS OF PHYSIOLOGY. 



impulses pass along the nerves supplying the respiratory muscles, 

 namely, the vagus to the muscles of the larynx, the cervical nerves to 

 the muscles of the neck, the intercostal nerves to the intercostal 

 muscles, and the phrenic nerves to the diaphragm. 



The centre is bilateral, each half controlling the muscles of the 

 corresponding- side, the two halves being connected by commissural 

 fibres. Its position has been determined by observing the effect upon 

 the respiratory movements of transection of the brain stem or spinal 

 cord at various levels. When a section is made across the pons or 

 medulla oblongata at any point above the level of the striae acusticae, 

 respiration is unaffected. If the spinal cord is divided at the upper 

 end of the cervical region, the respiratory muscles supplied by nerves 

 above the section, e.g. those which dilate the alse nasi, continue to 

 contract. Destruction of the medulla oblongata, in the region of the 

 apex of the calamus scriptorius, is at once followed by cessation of all 

 respiratory movements. The region occupied by the centre is not 

 sharply denned, but it is undoubtedly closely connected with the 

 sensory nuclei of the vagus nerves. 



There is no evidence of the existence of subsidiary centres in the 

 spinal cord. 



The centre continues to send out rhythmic impulses to the 

 respiratory muscles when it is cut off from afferent impulses reaching 

 it either from the higher parts of the brain or from the spinal cord. If, 

 however, the brain stem is divided below the pons, and the vagus nerves 

 are also divided, the respiratory movements are replaced by a series of 

 inspiratory spasms, and the animal dies after a short time. It is 

 doubtful, therefore, whether the centre can be regarded as acting 

 automatically in the absence of all afferent impulses, though this view 

 has been taken by some writers. 



The question is one of theoretical rather than practical interest, 

 since, in ordinary circumstances, a constant stream of afferent impulses 

 is reaching the centre and modifying its activity ; and hardly any 

 nervous centre in the body is more easily influenced in this way than 

 the respiratory centre. For instance, the mere directing of one's 

 attention to the respiratory movements is sufficient to alter their rate 

 or depth. The respiratory centre, like the vaso-motor centre, is also 

 extremely sensitive to any changes in the composition of the blood 

 supplying it. These two factors which modify its activity, namely, (1) 

 the composition j)f^ the blood, and (2) nervous impulses from the higher 

 centres or from the peripheral nerves, will be considered scpamtrlv. 

 The former affects primarily the depth, and the latter the rate of 

 respiration. 



