894 ANIMAL PHYSIOLOGY. 
1, If the phrenics, an intercostal nerve, etc., be cut, there is 
a corresponding paralysis of the muscle supplied. 2. If the 
spinal cord be divided below the medulla oblongata, there is a 
cessation of all respiratory movements except those of the 
larynx and face, which also disappear if the facial and recur- 
rent laryngeal nerves be divided. 3. So long as the medulla 
remains, respiration may continue; but if even a small part of 
this region, situated below the vaso-motor center between this 
and the calamus scriptorius (respiratory center, neud vital), 
be injured, death ensues rapidly. Plainly, then, there are cen- 
tral cells which originate the impulses that energize the mus- 
cles. 
It remains to inquire still whether they are independent 
(automatic) centers, or are influenced by impulses reaching 
them from without. Is the government absolute, or subject to 
the will of the multitudinous cells of the organic common- 
wealth ? 
Again let us appeal to facts: 1. If one vagus nerve be cut, 
a change is observable in the respiratory rhythm, which is 
much more pronounced if both nerves be divided. Respiration 
becomes slower, and the pause between inspiration and expira- 
tion greatly lengthened, though the gaseous interchange re- 
mains much as before. 2. If one suddenly step into a cold 
bath, he naturally draws a long breath. Again, the respiration 
is very greatly altered in consequence of emotional changes; 
indeed, there is probably no rhythm in the body more subject. 
to frequent obvious alteration than that of respiration. 3. 
Stimulation of the central end of such a nerve as the sciatic 
causes marked change in the rhythm of breathing. 4. Stimu- 
lation of the central end of the vagus usually quickens res- 
piration, while stimulation of the central end of the superior 
laryngeal has the opposite effect. If the current be strong, 
respiration may be arrested in each instance, though in a differ- 
ent manner. In the case of vagus stimulation the result is 
inspiratory spasm, and of the superior laryngeal expiratory 
spasm. 
These and a host of additional facts, experimental and other, 
show that the central impulses are modified by afferent im- 
pulses reaching the center through appropriate nerves. More- 
over, drugs seem to act directly on the center through the 
blood. 
The vagus is without doubt the afferent respiratory nerve, 
though how it is affected, whether by the mechanical movement 
