200 
RESPIRATION. 
are filled by the intercostal muscles, of which there are two 
layers, the fibres of the one layer decussating those of the other. 
Besides these muscles are the serrati, superficial costal, transverse 
costal, and levator costal, all of which are agents in the dilata- 
tion of the thorax. The chest thus formed constitutes a close 
cavity, but allowing the passage of the trachea, oesophagus, and 
bloodvessels. The inner walls are everywhere coated by the 
pleural membrane, and by it divided into two principal chambers, 
one for each lobe of the lungs ; and a third and subordinate one 
for the heart. Each of these principal cavities is occupied and 
filled by a most beautiful adaptation of the lungs ; each lobe 
having its apartment entire and circumscribed. 
The chest is an entire cavity : still the external atmospheric air 
has a free passage to its innermost recesses, from its entrance 
through the nostrils and nasal fossa, to the posterior chamber of 
the mouth. Here the air arrives at a very wonderful and truly 
surprising piece of mechanism, termed the larynx , composed of 
cartilages so constructed and arranged as to receive the opera- 
tions of a set of muscles, by which this portion of the respiratory 
passage can be dilated, contracted, and, if necessary, closed. It 
is here that the peculiar intonation of the voice is effected, and a 
safeguard placed against the passage of every element or consti- 
tuent, except atmospheric air, all others being foreign and noxious. 
Immediately posterior to the larynx the canal takes on a very 
different construction — it becomes a rounded tube, composed of 
cartilaginous rings, about fifty or sixty in number, called the 
trachea. The circle of these rings is not continuous, being sepa- 
rated behind by transverse and longitudinal fibres of elastic, or, 
perhaps, muscular tissue. This tube proceeds down the front of 
the neck, and, having arrived at the chest, passes between the two 
first ribs, and immediately divides into two trunks, called bronchi , 
one entering each lobe of the lungs, again dividing and subdividing 
to great minuteness, and ultimately terminating in small mem- 
branous vesicles whose diameters are said not to exceed the ^th 
of an inch. The inner surface of the whole of the respiratory 
passage is lined by a mucous membrane, commencing within 
the nostrils, continuing through the larynx and trachea to the 
utmost point of its divisions, and probably composing the air- 
cell itself. This mucous membrane is covered on its free surface 
by a beautiful epithelium formed of cells, to which vibrating cilia* 
are attached. 
The heart, as before stated, is situated in the third and lesser 
chamber of the thorax. This organ is of a conical shape, and has 
its base directed upward towards the spine, opposed to the 4th, 
5th, and 6th dorsal vertebrae. Its apex points downward and 
