600 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



quiescent, but that the respiratory centre, whose location we have 

 assumed to be in the medulla, is still sending out its motor impulses 

 is evidenced by the increased movements of the nostrils and glottis. If, 

 now, the facial and laryngeal nerves be cut this movement will also 

 cease. 



These facts indicate that in some part of the central nervous system 

 arise the motor impulses which are conducted to the muscles of respi- 

 ration. As one after the other of the paths of communication between 

 these centres and the muscles are interrupted, we find that one after the 

 other these muscles become paralyzed. 



It has been found, as already mentioned, that the brain may be 

 removed down to the level of the medulla oblongata and still respiration 

 take place, thus marking the upper limit of this centre. On the other 

 hand, the spinal cord may be divided up to the calamus scriptorius, and 

 through the movements of the face the evidence of the activity of the 

 respiratory centre may still be made out. The respiratory centre, there- 

 fore, lies some place between these two points. Its exact locality has 

 not been determined, but it is clear that it lies in the floor of the fourth 

 ventricle, somewhat lower down than the vaso-motor centre, and nearer 

 the tip of the calamus scriptorius. When this locality is injured, as by 

 puncturing with a sharp-pointed instrument, all respiratory movements 

 at once cease. The motions of the heart are, also, at once arrested from 

 a simultaneous irritation of the cardio-inhibitory centre. This point, 

 therefore, when injured causes instantaneous death, and was, conse- 

 quently, termed by Flourens the vital centre, the noeud vital. 



The action of this centre is automatic and is not reflex, though it 

 may be influenced by afferent impulses coming through various sensory 

 nerves. Thus, for example, a dash of cold water on the skin causes a 

 deep inspiration by exalting the action of this centre through the con- 

 duction of impulses to it from the external integument. 



Again, the activity of the respiratory centre may be influenced by 

 impressions coming from above. Thus, the various emotions, which may 

 be regarded as impulses originating in the cerebrum, also, are capable of 

 modifying its activity. The respiratory centre, while not dependent, 

 however, upon impulses coming through afferent nerves, is especially 

 modified in activity by impulses traveling through the pneumogastric 

 nerves; and though the pneumogastric nerves ma} T be regarded as the 

 principal paths of conduction of afferent impulses, modifying the activity 

 of the respiratory centres, their integrity is not essential for the mani- 

 festation of the peculiar action of this ganglionic centre. This fact.is 

 demonstrated by the modifications produced in respiration by section of 

 these nerves. 



If one pneumogastric nerve be cut respiration becomes slower and 



