THE RESPIRATORY SYSTEM, 897 
are both inspiratory and expiratory centers in the spinal cord. 
But, as we have pointed out, on more than one occasion, we 
must always be on our guard in interpreting the behavior of 
one part when another is out of gear. There is so much latent 
resource, so great a power to resume functions normally laid 
aside, if not wholly in great part, that we should hesitate be- 
fore inferring that the spinal cord usually takes a prominent 
share in originating the impulses which govern respiration. 
Notwithstanding the suggestiveness of such experiments, we 
do not think they make the medulla appear in a less important 
light as the part of the nervous system dominant in respira- 
tion; though there may be nervous machinery in the cord usu- 
ally in feeble action, susceptible of assuming a more exalted 
functional réle when occasion urgently demands and when en- 
couraged, so to speak, to do so, as in the experiments referred 
to above; indeed such, upon our own theory of physiological 
reversion, would naturally be the case. We must, however, 
draw the line between what is and what may be in function. 
The Influence of the Condition of the Blood in Respiration.—If 
for any reason the tissues are not receiving a due supply of 
oxygen, they manifest their disapproval, to speak figuratively, 
by reports to the responsible center in the medulla, and if the 
medulla is a sharer in the lack, as it naturally would be, it takes 
action independently. One of the most obvious instances in 
which there is oxygen starvation is when there is hindrance to 
the entrance of air, owing to obstruction in the respiratory 
tract. 
At first the breathing is merely accelerated, with perhaps 
some increase in the depth of the inspirations (hyperpnea), a 
stage which is soon succeeded by labored breathing (dyspnea), 
which, after the medulla has called all the muscles usually em- 
ployed in respiration into violent action, passes into convul- 
sions, in which every muscle may take part. 
In other words, the respiratory impulses not only pass along 
their usual paths as energetically as possible, but radiate into 
unusual ones and pass by nerves not commonly thus set into 
functional activity. 
It would be more correct, perhaps to assume that the vari- 
ous parts of the nervous system are so linked together that ex- 
cessive activity of one set of connections acts like a stimulus to 
rouse another set into action, the order in which this happens 
depending on the law of habit—habit personal and especially 
ancestral. An opposite condition to that described, known as 
