OF THE BRAIN AND l^ERVES. 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 volume 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 j)rolonged as they contract 

 into the olfactory lobule, so that this part is placed on a 



