306 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



the inspiratory muscles are completely relaxed. Towards the end of 

 the pause, active contraction of the expiratory muscles takes place. 

 These spasms are inadequate for the proper aeration of the blood. As 

 the blood gets more venous, the spasms become less and less frequent, 

 and the animal, at the end of an hour or so, dies of asphyxia. We must 

 conclude from these results that, in default of the normal vagus 

 stimulation, the activity of the respiratory centre can be regulated and 

 controlled to a large extent by vicarious influences from the higher parts 

 of the brain. In searching for the origin of these impulses, we naturally 

 think first of the afferent nerves which enter the cerebro-spinal centres 

 above the medulla oblongata, especially since we know that respiration 

 may be altered in an inspiratory or expiratory direction by excitation of 

 the olfactory, optic, auditory, and fifth nerves. A total division of these 

 nerves would be probably impracticable. Loewy 1 has shown that 

 bilateral section of the fifth nerves at their exit from the pons does not 

 alter the respiratory rhythm after division of both vagi. Attempts 

 have therefore been made to localise the origin of these impulses 

 first, by carrying out the extirpation of the brain in successive stages 

 from above downwards ; and, secondly, by observing the effects of 

 stimulation of certain parts of the higher portions of the brain. 



By the first method it has been determined that, after division of 

 both vagi, no further alteration takes place in the breathing until the 

 section has been carried through the level of the anterior corpora 

 quadrigemina. 



By the second method, certain localised areas of the brain have been 

 found to give, on excitation, definite changes in the respiratory rhythm. 

 Thus mechanical, thermal, or electrical stimulation of the inner wall 

 of the optic thalamus on each side (Budge, Christiani) causes either a 

 standstill of the diaphragm in an inspiratory condition, or increases 

 the rhythm and depth of the inspiratory movements. On the other 

 hand, an expiratory centre has been described by Christiani 2 in the 

 substance of the anterior corpora quadrigemina close to the aqueductus 

 Sylvii, and an inspiratory centre by Martin and Booker 3 midway 

 between the anterior and posterior corpora quadrigemina. It would seem 

 that a section has to pass through or behind these two last-named 

 centres, in order to produce the effects already described of cutting off 

 the higher centres from the medulla oblongata after division of the vagi. 



Spencer 4 obtained slowing and arrest of respiration by faradic 

 stimulation of the cortex just outside the olfactory lobe in front of the 

 point where the tract joins the temporo-sphenoidal lobe, and increased 

 action on stimulating a spot on the convex surface of the cortex within 

 the motor area. Excitation of the junction of the olfactory bulb 

 and tract, as well as of the uncinate gyrus, produced super-inspiratory 

 clonus, while super-inspiratory tonus was obtained on stimulating the 

 descending motor tract. 



The experimental facts at our command are not adequate at present 

 to permit of our arriving at any definite conclusions as to the nature 

 or mode of origin of the vicarious impulses which reach the medulla 

 oblongata from these centres. We have, however, no reason to suppose 



1 Arch.f. PhysioL, Leipzig, 1893, S. 185. 



2 Monatsb. d. k. Akad. d. Wissensch. zu Berlin, 1881, S. 213. 



3 Journ. PhysioL, Cambridge and London, 1879, vol. i. p. 370. 



4 Phil. Trans., London, 1891, vol. clxxxii. p. 201. 



