OF THE BRAIN AND NERVES. 
65 
wards the heart, forms a very narrow canal, and descends 
behind the stomach, where it again enlarges into a very ca- 
pacious sac, the end of which extends to the anus, lodging 
itself between the inferior spinous processes of the tail. 
The air-cells in this are far less numerous than in the for- 
mer species, although they accompany the trachea, from its 
origin to the point where it forms that spacious appendage, 
which probably also supplies the place of a swimming- 
bladder. 
This arrangement of the respiratory organs, varying in 
the different species of Ophidians, demonstrates that the form 
of the organs has an important influence on the exercise of 
their functions ; and that it is erroneous to deduce characters 
for the classification of serpents, from these anomalies of 
form, or to regard the species in which the lung is divided 
into two lobes, as consuming a larger volume of oxygen^ 
and consequently as possessed of a more perfect organiza- 
tion. 
The principal air-cells are sometimes supported by nar- 
row cartilaginous slips, which are lost in the minute tubes 
forming that part of the pulmonary tissue where oxygena^ 
tion goes forward. The upper end of the larynx is sur- 
mounted by the two arytenoid cartilages, which leave 
between them a simple longitudinal fissure, the glottis. 
This simple apparatus, moved by two pairs of muscles, 
represents in Ophidians the organ of voice, which, how- 
ever, consists merely in hissing, more or less acute, 
produced by the air forced from the lungs. The opening 
corresponds to the interior aperture of the nostrils, and 
is more or less approximated to the end of the snout,, 
according as the species frequents the water or the land. 
OF THE BRAIN AND NERVES. 
The smallness of the brain of Ophidians is especially 
conspicuous, when we compare the valume of that organ 
with that of the head, in species in which the organs of 
mastication have acquired their fullest development. In 
front, the two hemispheres are prolonged as they contract 
into the olfactory lobule,, so that this part is placed on a 
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