NERVOUS MECHANISM OF RESPIRATION 167 



fort of the will that the movements are kept up. They belong 

 to the class known as automatic ; that is, they are, up to cer- 

 tain limits, under the control of the will, but recur in a reg- 

 ular, coordinate and orderly manner without the active inter- 

 vention of volition. Respiration represents the activity of a 

 self-governing apparatus. These movements constitute a 

 finely coordinated set of contractions contractions which 

 are regulated by means of afferent and efferent nerves under 

 the supervision of the respiratory center. 



The respiratory center is in the lower part of the medulla 

 oblongata. Destruction of the encephalon above, or the cord 

 below, the center does not arrest respiration. It is bi- 

 lateral a center for each side and these are more or less 

 independent of each other, but are so intimately connected 

 by commissural fibers that any impression made upon one 

 usually produces a like effect upon the other. Each half pre- 

 sides over the lungs and respiratory muscles of its own side, 

 but acts synchronously with its fellow of the opposite side. 

 Furthermore, each of these lateral centers may be regarded 

 as consisting of two parts, one for inspiration and one for 

 expiration. Stimulation of the inspiratory center not only 

 strengthens the inspiratory act, but also accelerates respira- 

 tion. Stimulation of the expiratory center strengthens ex- 

 piration and also retards the respiratory rate. The acceler- 

 ator portion of the center seems more sensitive than the in- 

 hibitory, and the result of stimulation of the whole center is t 

 therefore quickened respiration. 



Subsidiary respiratory centers are said to exist in the tuber 

 cinereum, optic thalamus, corpora quadrigemina, pons Va- 

 rolii and spinal cord ; but the existence of at least some of 

 these is doubtful. 



Rhythm of Respiration. 'What agency excites the center 

 to keep up the respiratory movements with such regularity 

 is a matter of interest. The chief circumstances which seem 

 to affect the rate and rhythm are (i) the will, (2) emotions, 

 (3) composition of the blood, and (4) afferent impressions. 



