386 CHORDATA. 
anterior mesenteric and posterior mesenteric arteries, which 
should be identified (see page 388) 
The cesophagus may then be cut through near the dia- 
phragm and the rectum near the anus, and if the mesentery 
be carefully cut through the whole alimentary system may 
be removed and unravelled. The thoracic cavity should 
now be opened by cutting through the ribs on either side 
and between the diaphragm and the sternum. The cavity 
is almost entirely filled by the two lungs and the heart. The 
trachea can be traced down the neck (see Plate XIII.). 
Just where it emerges from the buccal cavity there is a 
cartilaginous /arynx which forms the organ of voice. It 
is formed of thyroid and cricoid cartilages modified from 
branchial arches in the embryo (see page 417). The trachea 
throughout its course is distended by cartilaginous rings. It 
passes into the thoracic cavity anteriorly and divides into 
two bronchi which lead to the lungs in which they branch. 
(These are best seen on removal of the blood-vessels.) 
The Jungs are of a bright-red colour, spongy, and lying 
quite free in the cavity around them. The left lung has two 
Respiratory lobes, the right has four. Each lung is envel- 
* oped by a layer of peritoneum called the /eura, 
which has the same relationship to the lung as has the 
pericardium to the heart. The outer layer of the pleura is 
pushed against the ribs and the inner adheres to the lung. 
Between the two is the pleural cavity, which is practically 
squeezed out of existence in the living animal by the ex- 
pansion of the lungs. Between the two pleura is a space, 
the mediastinal space, nearly filled by the heart and peri- 
cardium. 
Hence the perivisceral coelom in the rabbit is divided 
into no less than four separate parts—the pericardial 
cavity, two pleural cavities and the abdominal 
cavity. Between the last and the other three 
is the diaphragm. The diaphragm is innervated by a pair 
of phrenic nerves arising from the fourth spinal nerve in 
the neck. They may be easily seen passing down between 
heart and lungs. The capacity of the thorax is increased by 
raising of the ribs, caused by contraction of the intercostal 
muscles and by the lowering of the diaphragm. Air is 
in this way inspired or drawn into the lungs. Expiration 
Colom. 
