530 RESPIRATION 



to respiration, its isolation and subsequent paralysis would make life 

 practically impossible. This is especially true of young animals. 

 For this reason, it has been advocated to regard the various nuclei of 

 the nerves innervating the different muscles of respiration, as secondary 

 or tributary centers to the main or medullary center of respiration. 

 It does, however, seem scarcely necessary or helpful to look at the 

 respiratory mechanism in this way, because in reality these different 

 nuclei form nothing more than mere stations upon the efferent path and 

 do not possess automatic power. The fact that the respiratory center 

 is situated in the medulla, may also be proved by injuring this structure 

 directly, as may be done by introducing a pointed instrument between 

 the adjoining dorsal borders of the atlas and axis. This constitutes 

 the act of pithing, a procedure which leads to an almost instantaneous 

 stoppage of respiration and a loss of the vascular tonus on account of 

 the destruction of the vasomotor center. The cardiac center is also 

 involved, although the heart itself continues to beat for a brief period 

 of time. Very similar conditions may be produced by sharply bending 

 the head upon the trunk, in which case the odontoid process of the axis 

 may lacerate the bulbar tissue. 



The Cause of the Activity of the Respiratory Center. — The foregoing 

 discussion has shown that the respiratory motions are incited at regu- 

 lar intervals by impulses sent out by the center. The question which 

 now presents itself is this: Does this center possess the power of 

 discharging these rhythmic impulses in consequence of an inherent 

 property of its constituents, or does its activity depend upon afferent 

 impulses conveyed to it from other parts of the body? In brief, there- 

 fore, it would be necessary to ascertain whether the cells of the respira- 

 tory center possess an automatic power, such as is exhibited by the 

 components of the cardiac center, or whether they are activated solely 

 in a reflex way. 



It must be conceded that the former view is the correct one, i.e., 

 the rhythm is inherent in these nerve cells and is not generated in a 

 reflex manner. This conclusion is based upon the fact that the center 

 may be completely isolated from the rest of the bodj^ by the division 

 of its afferent connections without producing an absolute cessation of 

 the respiratory movements. An experiment of this kind necessitates 

 the division of the brain stem above the medulla and the severance of 

 the vagi and glossopharyngeal nerves. In addition, the spinal cord 

 must be cut across below the nuclei of the phrenic nerves,^ and must 

 also be rendered impermeable to sensory impulses by dividing the pos- 

 terior roots in its cervical portion. But even now the objection might 

 be raised that the center cannot be considered as being completely 

 isolated as long as it remains in connection with such efferent nerves 

 as the phrenics, the probabihty being that these nerves also conduct 

 in a centripetal direction. This contention has been disproved in 

 the following way. Having thoroughly curarized an animal in order 

 1 Loewy, Pfliiger's Archiv, xlii, 1889, 245. 



