NORMAL STIMULATION OF THE RESPIRATORY CENTER 331 



the same effect. After destruction of the semicircular canals respiration becomes 

 slower and deeper. According to Marckwald, the glossopharyngeal produces 

 respiratory standstill in whichever phase the respiratory center happens to be 

 overtaken by the stimulus, but according to others it behaves just like the other 

 cutaneous nerves. The last named have an inspiratory effect with weak stimu- 

 lation, and an expiratory effect with strong stimulation. The phrenic also con- 

 tains afferent fibers which appear to act like the cutaneous nerves, and other 

 afferent muscular nerves behave on the whole like these. With regard to the 

 sympathetic nerves it is stated that the splanchnic causes only an expiratory 

 contraction and that the cervical sympathetic influences both phases of respira- 

 tion. Finally, by stimulation of the heart and of the aorta, reflex respiratory 

 movements and contractions in the air passages have been obtained. 



4. NORMAL STIMULATION OF THE RESPIRATORY CENTER 



Seeing then that the respiratory center is reflexly influenced by the most 

 widely different afferent nerves, it would be natural to suppose that it is 

 roused to action only in the reflex manner. But this conclusion is not 

 warranted. We have already seen indeed that the respiratory center isolated 

 from the brain pathways on a vagotomized animal is still very powerfully 

 active. This might be due partly to the stimulating effect of the section and 

 partly to the afferent impulses still reaching the center. But respiratory 

 movements continue when the cerebrum is extirpated, the vagi cut and the 

 spinal cord sectioned below the exit of the respiratory nerves (Rosenthal). 

 It can scarcely be assumed that the respiratory movements are called out by 

 the few afferent impulses remaining after all these operations. Besides there 

 are still other facts which tend to prove that the excitation of the respiratory 

 center is attributable mainly to the properties of the blood. 



The fcetus in the uterus does not breathe: respiration begins only after 

 birth. What is the cause of the very first act of respiration? The blood of the 

 fo3tus is arterialized, so long as the placental circulation is maintained, at the 

 expense of the mother's blood. The temperature of the amniotic fluid in which 

 the foetus is submerged is exactly the same as that of the foetus itself, so that 

 it is not subjected to any temperature stimuli nor to any other cutaneous 

 stimuli. At birth the circumstances of life change suddenly: the placental cir- 

 culation ceases and the skin is subjected to different sensory stimuli. The cause 

 of the first act of respiration is to be sought therefore either in the cessation of 

 the placental circulation or in the sensory stimulation of the skin. 



Both these possibilities have their advocates. But from the present infor- 

 mation it would seem that the discontinuance of the placental circulation is 

 the real determining factor. It is true that one can produce various motor 

 reflexes, and respiratory movements among them, by means of cutaneous stimuli 

 applied while the placental circulation is still continuous. But such responses 

 are both infrequent and temporary in character. Besides, cutaneous stimulation 

 may be kept up for a long time without ever a sign of a respiratory movement. 

 Contrast with this the result of destroying the placental circulation. However 

 the experiment be performed, whether by clamping or bleeding the umbilical 

 cord, or, the uterus being undisturbed, by poisoning the mother with carbon 

 dioxide or, finally, by bleeding the mother, respiratory movements of the fcetus 

 are always obtained. 



