82 RESPIRATORY CENTRE. 



The spinal cord is entirely insensible to mechanical 

 stimulation excepting in the immediate neighbourhood of 

 its motor roots. Its grey substance seems to be also 

 wholly insusceptible of electrical stimulation, but its fibres 

 can be excited either by single induction shocks or by 

 faradization. 



Centres of the Medtdla Oblongata. 



The central canal of the cord opens out into the rhomboidal space, or 

 fourth ventricle, the two grey columns (horns) thus becoming superficial, and 

 assuming such a position that what was before posterior lies outside. In the 

 stratum of grey substance thus exposed, are contained the regulatory centres 

 which preside over the most important functions of the body, namely those of 

 the heart, of the arteries, of the respiratory organs, of the organs of digestion, 

 of speech, of taste, and of locomotion. The origins of the nerves concerned 

 in the functions of these centres are in close relation with each other, but 

 nothing precise is known of their anatomical relations. 



The regulatory centre for the heart is represented by two tracts of grey 

 substance on either side of the spinal canal, but nearer to the posterior sur- 

 face. At the cal. script, these diverge and become continuous with the vagal 

 tracts (alee cinerece) which are separated from each other by the nuclei of the 

 hypoglossal nerve. Each vagal tract is in relation at its upper end with the 

 nucleus of the glossopharyngeal nerve, which is close to the auditory striae. 

 Outside of each vagal tract are the internal and external nuclei of the auditory 

 nerve, which are respectively continued downwards into the grey tubercle of 

 Rolando and the restiform nucleus. Higher up, the internal auditory nucleus 

 becomes continuous with the origin of the sensory division of the trigeminus, 

 the motor division of which springs from the grey substance nearer the middle 

 line. The hypoglossal nucleus is continuous with those of the abducens and 

 facial, which lie underneath the eminentia teres of each side, and is in relation 

 externally with the origin of the motor root of the trigeminus. The same 

 motor tract is continuous upwards with the grey substance underneath the 

 floor of the aqueduct, from which the oculomotorius and trochlearis spring. 



Influence of the Nervous System on respiration. The 

 respiratory nervous system consists of (a) the regulatory 

 centre (vagal tracts) ; (b} the afferent fibres of the vagus ; 

 (c) motor fibres contained in the facial and recurrent, as 

 well as in the phrenic, intercostal, and other spinal nerves, 

 (i.) Destruction of the vagal tracts produces instant death 

 in mammalia ; destruction of the upper part only, arrests 



