The nerve-centres which are implicated in quiet respiration are situated in that part of the brain termed the medulla oblongata, at the point where the vagi nerves take their origin. The destruction of this very small spot causes immediate death by arrest of the respiratory acts. It was accordingly named the " nceud vital", or vital spot, by Flourens. Nerve fibres run to and from this point. Amongst those which convey nervous impulses to it are the fibres of the vagus nerve which pass up to it. Those which conduct impulses away from it are fibres which pass down the spinal cord for some distance, and then emerge at the lower part of the neck and along the thorax to form the phrenic and intercostal nerves distributed to the diaphragm and the intercostal muscles.
When at rest, the horse breathes ten or twelve times per minute, and there is a general relation between the number of respirations and the number of the beats of the heart, the proportion being about 1:4 or 1:5, but the frequency with which the acts of respiration are performed is subject to great variation. At rest, or during sleep, the number is about ten or twelve in the minute, but after vigorous exercise, such as galloping for ten minutes, it may rise to fifty, sixty, or more in the same time, gradually subsiding as the animal becomes quiescent. In two horses, after a run of about 7 miles, the pulse was observed to rise from 40 to 132, and the respirations from 12 to 102, in the minute. After being at rest for three-quarters of an hour the pulse had fallen to 66 and to 54, whilst the respirations in each animal were 60 per minute (Arloing). The depth of the respirations greatly augments with increased frequency, so that a much larger volume of air enters, and is expelled from, the lungs at each inspiration and expiration. This constitutes forced respiration.