THE RESPIRATORY SYSTEM. 393 
entrance of air,etc. Even as it is, the need of a constant supply 
of oxygen in warm-blooded animals is much greater than in 
cold-blooded creatures, which can long endure almost entire 
cessation of both respiration and circulation, owing to the com- 
paratively slow rate of speed of the vital machinery. 
Tf one were to rely on mere appearances he might suppose 
that in the more active condition of certain organs there was 
less chemical interchange (respiration) between the blood and 
the tissues than in the resting stage, or, properly speaking, 
more tranquil stage, for it must be borne in mind that a living 
cell is never wholly at rest; its molecular changes are cease- 
less. It happens, e. g., that when certain glands (salivary) are 
secreting actively, the blood flowing from them is less venous 
in appearance than when not functionally active. This is not. 
because less oxygen is used or less abstracted from the blood, 
but because of the greatly increased speed of the blood-flow, so 
that the total supply to draw from is so much larger that, 
though more oxygen is actually used, it is not so much missed, 
nor do the greater additions of carbon dioxide so rapidly pol- 
lute this rapid stream. 
It is thus seen that throughout the animal kingdom respira- 
tion is fundamentally the same process. It is in every case 
finally a consumption of oxygen and production of carbonic 
anhydride by the individual cell, whether that be an Amceba 
or an element of man’s brain. These are, however, but the 
‘beginning and end of a very complicated biological history of 
by far the greater part of which nothing is yet known; and it 
must be admitted that diffusion or any physical explanation 
carries us but a little way on toward the understanding of it. 
THE NERVOUS SYSTEM IN RELATION TO RESPIRATION. 
We have considered the muscular movements by which the 
air is made to enter and leave the lungs in consequence of 
changes in the diameters of the air-inclosing case, the thorax. 
It remains to examine into the means by which these muscles 
were set into harmonious action so as to accomplish the pur- 
pose. The nerves supplying the muscles of respiration are de- 
rived from the spinal cord, so that they must be under the 
dominion of central nerve-cells situated either in the cord or 
the brain. Is the influence that proceeds outward generated 
within the cells independently of any afferent impulses, or is it 
dependent on such causes? Let us appeal to facts. 
