396 ‘ ANIMAL PHYSIOLOGY. 
tory center, they are probably influenced by the respiratory 
condition of the blood, though not necessarily exclusively. 
But when all the principal afferent impulses are cut off by 
division of the nerves reaching the respiratory center directly 
or indirectly, respiration will still continue, provided the motor 
nerves and the medulla remain intact. 
The center, then, is not, mainly at least, a reflex but an auto- 
matic one, though its action is modified by afferent impulses 
reaching it from every quarter. Since respiration continues 
when the medulla is divided in the middle line, yet is modified 
unilaterally when one vagus is divided, it is inferred that the 
respiratory center is double, that each half usually works in 
harmony with the other, but that each can act independently. 
Though it seems clear enough that the respiratory center is 
automatic, and that its action is modified according to the con- 
dition of the organism generally, as communicated to it by the 
various afferent nerves and the blood itself, yet the exact man- 
ner of its action—why inspiration follows up expiration—has 
not been clearly explained. Some assume that during expira- 
tion inspiratory impulses are gathering head and finally check 
expiration by originating inspiration, while these are opposed 
by another process which at length gives rise to enough resist- 
ance to check inspiration, and originate expiration; and this 
theory becomes more complete if an expiratory as well as in- 
 spiratory center be assumed. 
We have hitherto spoken only of a single respiratory cen- 
ter in the medulla, but certain experimental facts throw addi- 
tional light on the subject. 
In young mammals—e. g., kittens—it is found that, in the. 
absence of the medulla, respiratory movements may be induced 
by stimulating (pinching) the surface, especially if the action 
of the spinal cord be augmented by the administration of 
strychnia. From this it has been inferred that there are respir- 
atory centers in the spinal cord, subordinate to the main cen- 
ter in the medulla. Considering the imperfect nature of the 
respiratory act as thus induced, and the circumstances of the 
case, the conclusion has the appearance of being a little strained. 
But quite recently it has been shown that in the adult dog 
when the cord is severed below the medulla, and artificial res- 
piration maintained for some time, on ceasing this, breathing 
begins spontaneously and continues for a considerable period; 
and the expiratory phase of respiration in this case is the most 
marked. It has been argued from this experiment that there 
