33 



THE NERVOUS SYSTEM 



The Respiratory Reflex Mechanism. The maintenance of the normal res- 

 piratory rhythm is dependent upon a respiratory center in the caudal part of 

 the medulla oblongata, which is sensitive to changes in the carbon dioxid con- 

 tent of the blood. But this rhythm is also influenced by afferent impulses coming 

 from the lungs by way of the vagus nerve and the tractus solitarius. It is 

 probable that these impulses are relayed through the nucleus of the tractus soli- 

 tarius and descending fibers that arise in that nucleus (tractus solitariospinalis) 

 to the primary motor neurons belonging to the phrenic and intercostal nerves 

 (Fig. 245). There must also be a descending tract from the respiratory center 

 to these neurons. Cajal (1909) believes that this center is, in fact, identical 

 with the lower part of the nucleus of the tractus solitarius (the commissural 



Dorsal motor X nucleus 

 Nucleus offascic. solitarius 



Fasciculus solitarius 



Vagus ganglion 



Vagus nerve 



Tr. solitario-spinalis 



Sympathetic ganglion 



Blood-vessel 

 Respiratory center 



Intercostal nerve 



Intercostal muscle 



Phrenic nerve 



Diaphragm 

 Fig. 245. Reflex mechanism of respiration. (Herrick, Cajal.) 



nucleus), and that this responds both to changes in the chemical composition of 

 the blood and to the afferent impulses coming by way of the vagus nerve. If 

 this be true, the fibers from the nucleus of the tractus solitarius would be the 

 only descending tract needed to carry the respiratory impulses to the spinal 

 cord. Although on its afferent side the respiratory reflex is visceral, it is ex- 

 ecuted by somatic muscles which are under voluntary control; and hence breath- 

 ing may be temporarily suspended or the rhythm altered at will. 



The reflex mechanism for vomiting and coughing is illustrated in Fig. 246. 

 As the result of an irritation of the gastric mucous membrane a wave of excitation 

 travels along the afferent fibers of the vagus nerve and the tractus solitarius. 

 After passing through synapses in the nucleus of that tract, the impulses probably 



