Page:The Harveian Oration,1902.djvu/28

 deliquium animi by voluntary compression of the chest and forcible expiration with the glottis closed. This exerts such pressure on the great veins as completely to obstruct the flow of blood through the heart and to cause it to stop for several beats. Some of the cases of apparently direct voluntary control over the heart's action may be, therefore, accounted for through the medium of the respiratory mechanism; but there are others in which the rate of the heart has been capable of acceleration at will, independently of the respiration and without calling up special ideas. In all such instances there has been also unusual power over the voluntary muscles.

Like the heart, the calibre of the blood-vessels is regulated by mutually antagonistic sets of nerves—the vaso-constrictors (vaso-motor) and the vaso-dilators (vaso-inhibitory). That the lumen of the blood-vessels was subject to variation under the influence of the nervous system was postulated long before it was actually demonstrated. The credit of this prevision has been assigned to various writers.