182 
ANIMAL PATHOLOGY. 
most of all by the action of certain muscles, the diaphragm the 
chief of them, and also the intercostals and the levators of the 
ribs, and the pectorales and the serrati magnus et posticus, and 
others. By means of these the cavity of the thorax is enlarged, 
the pressure on the surface of the lungs is removed, and, on the 
common principle of pneumatics, the air which is pressing on 
every side and on every part enters the nostrils, and passes down 
the trachea, and occupies once more the bronchi and the air-cells. 
The Excito-motory System . — The nerves by which these 
auxiliary systems of muscles are governed, arise not from the 
same source as those which direct the contraction of the bronchial 
muscles, and have no direct connexion with them ; affording 
another illustration of that system of sympathetic nervous influ- 
ence which so satisfactorily explains many of the phenomena 
of life. There are certain muscles which are devoted to one 
special service ; their number, however, has rapidly diminished 
since the researches of the physiologist and the pathologist have 
been so diligently and so successfully directed to the elucidation 
of the nervous system. There are certain nerves which usually 
are placed under the perfect control of the will : but at other 
times, without the command of the will, and in despite of it, are 
pressed into the service of organic life. Some of them performed 
most important offices before the will began to exert its power ; 
and they now continue to be voluntary or involuntary muscles 
according as the powers of life demand their aid, or the will 
slumbers with regard to them. The most important functions 
of life are performed far more by these semi-voluntary muscles 
than by those which belong exclusively to organic life. We 
have had a striking instance of this in the bronchial muscles. 
Without them the air would have been imperfectly expressed 
from the central portions of the lungs ; but their strength alone is 
weakness, and the effect absolute imperfection, compared with the 
assistance derived from the nerves of voluntary power. This 
connexion of nervous influence — this subserviency of the volun- 
tary muscles to the purposes of life, prevents the undue accu- 
mulation of muscles, and the unseemly and inconvenient increase 
of bulk, without the slightest sacrifice of the comfort or welfare of 
the individual. The bronchial nerves are acted upon by the 
stimulus of the carbonic acid which is gathering in the tubes 
and the cells of the lungs. They are excited to their most ap- 
parent action — they cause the tubes and the air-cells to contract 
upon and to expel a portion of the air. While, however, they 
are doing this, the effect of the stimulus by which they were 
animated is conveyed to the source whence they sprung — the 
medulla oblongata, — and is reflected thence to the origins of other 
