152 



THE MECHANISM OF THE CIRCULATION. 



1. On excitation of the central end of the divided vagus in a 

 curarised dog, the pressure in the pulmonary vein fell, in the 

 pulmonary artery rose, in the aorta fell. These results, brought about 

 by retiex excitation of the vasomotor centre, seem to point conclusively 

 to constriction in the pulmonary arterioles. 



2. After section of both vagi in a curarised dog, excitation of the 

 peripheral end of the spinal cord, divided at the level of the seventh dorsal 

 root, produced a rise of aortic pressure equal to 160 mm. Hg. During 

 the period of stimulation, the pulmonary arterial pressure rose 1 mm. 

 Hg (Fig. 91). On the other hand, excitation of the vasomotor centre 

 immediately afterwards produced a rise of 57 mm. Hg in the aorta, and 

 6 mm. Hg in the pulmonary artery (Fig. 92). 



The rise of pressure by 6 mm. Hg, in the latter case, cannot well be 

 explained by a passive back effect, because, just before, a rise of aortic 

 pressure thrice as great had only raised the pressure in the pulmonary 

 artery 2 mm. Hg. Since the vagi were divided this experimental 

 result cannot be explained by change in the rate of the heart. 



Carrying the investigation a step further, these authors divided the 



upper thoracic nerve 

 roots, and stimulated 

 the peripheral ends. 

 Accepting the ana- 

 tomical researches 

 of Gaskell, it is from 

 this part of the 

 spinal cord that wo 

 should expect to find 

 the outflow of the 

 pulmonary vaso- 

 constrictor nerves. 

 Effect Excitation of the 

 third thoracic root 

 accelerated the heart, raised the tension in the pulmonary artery at 

 the same time, and frequently caused a slight fall in aortic pressure. 

 Here the aortic fall is neither produced by loss of cardiac power, nor 

 by dilatation of any vascular area, and must be ascribed to the 

 diminished input, consequent on contraction of the pulmonary 

 arterioles. 



On excitation of the succeeding roots, the proportionate change 

 in the aorta and pulmonary artery were found to be as follows : — 



Fig. 92.— Continuation of experiment shown in Fig. 91 

 of excitation of vasomotor centre. — Bradford and Dean. 



These results show conclusively that the pulmonary rise is not 

 produced passively. On passing down from root to root, the excitation 

 produces less effect on the pulmonary vascular system, and more on the 

 systemic system, for the splanchnic fibres become stimulated towards 

 the middle thoracic region. 



3. On producing asphyxia in the curarised animal, Bradford and 

 Dean observed the following : — (1) A great rise in aortic tension, followed 



