326 RESPIRATION 



for only three-quarters of an hour. Often it cannot be induced at all. The 

 animal reacts unusually well to all kinds of sensory stimuli causing reflex mus- 

 cular contractions, and the spinal vasomotor centers react powerfully to the 

 stimulus of asphyxiation. The effect of shock therefore is past; and yet as a 

 rule one observes no genuine respiration. To maintain the doctrine of the pre- 

 ponderance of spinal respiratory centers under such circumstances, one must 

 assume that these centers react toward shock in quite another way from the 

 other spinal centers. 



The fact that hemisection of the spinal cord very often does not result in 

 cessation of the respiratory movements of the same side (Brown-Sequard 

 et al.} speaks against the hypothesis of shock. Moreover, when cessation does 

 occur, it is immediately nullified if the phrenic of the opposite side be cut 

 (Porter). If the mechanical injury of sectioning were to produce so strong 

 a shock, as the advocates of the spinal centers assume, hemisection of the 

 spinal cord should stop the respiratory movements on the side of the section 

 for a time at least. 



We reach the conclusion therefore that the medulla is not only of great 

 importance in the regulation of respiratory movements, but that it controls 

 also the coordinated activity of the respiratory muscles. Only in rare cases 

 is such an effect carried out by the nuclei of the spinal cord, and, although 

 we can speak in general terms of spinal respiratory centers, it appears that 

 in comparison with those of the medulla they have but little to do with 

 producing the normal stimuli. 



The exact location of the respiratory center in the medulla is not yet 

 definitely known. This much appears certain, however, that it is not a small, 

 circumscribed spot, but is a region of relatively large extent. This is what 

 we should expect in view of the very large number of nervous connections 

 which it has. 



After median section of the medulla, the respiratory movements of the two 

 halves of the diaphragm (Langendorff) and those of the vocal cords and the 

 nose (Kreidl) continue synchronously which shows that the influences origi- 

 nating the respiratory movements proceed simultaneously on the two sides of 

 the center. But this synchronism is abolished by section of both vagi, and each 

 half of the body then breathes independently of the other. 



That section of the vagus on one side does not always stop the synchronism 

 just mentioned, goes to show that the two centers are connected by commissural 

 fibers. The presence of a crossed connection between the respiratory center and 

 the nuclei of the respiratory muscles follows also from the above-mentioned 

 facts, that respiration can proceed undisturbed on one side after hemisection of 

 the cord on that side and section of the phrenic on the opposite side. 



The respiratory movements can be influenced also by stimulation of parts 

 of the brain anterior to the medulla. Martin and Booker obtained inspiratory 

 effects by stimulating the surface of a section between the anterior and posterior 

 corpora quadrigemina ; Christiani obtained the same on stimulation of the 

 floor of the third ventricle, and expiratory effects on stimulation of the 

 entrance of the aqueduct of Sylvius. Finally, the cerebral cortex evidently 

 exercises control over the respiratory movements, as is seen, for example, 

 in the extremely fine gradations of these movements which can be executed 



