FISHES. 379 



The vertebrae of fishes are united together by concave sur- 

 faces, filled with cartilage. In the greater number these vertebrae 

 have long spinous processes which keep the body in a vertical 

 position. The ribs are often joined to transverse processes. 



Though the head in fishes varies more in point of form 

 than in any other class of animals, it always consists of the 

 same number of bones. The frontal bone is composed of six 

 pieces ; the parietal of three ; the occipital of five ; the sphenoi- 

 dal bone of five, and each temporal bone of two pieces. The 

 cranium forms but a small portion of the head. The brain is 

 enveloped by a gelatinous matter, and forms many ganglions 

 or consecutive knots, as in the reptiles ; and there are ganglions 

 or knots, besides, at the base of the olfactory nerve. A superficial 

 nerve also runs along the body, almost immediately under what 

 is called the lateral line, from the head to the tail. Sensation 

 appears to be weak in almost all the class, although many, as 

 the eel, possess irritability after being cut into small portions. 



The nostrils in fishes are simple cavities or hollows at the 

 point of the snout, in the interior of which are disposed laminae 

 in a radiated form. These cavities are often divided into two 

 compartments, and sometimes, as in the lamprey, the two nostrils 

 are united into one. The eye is possessed of a very flat cornea, 

 with but little aqueous humour, but the crystalline lens is al- 

 most globular and very hard. In general the eyes of fishes are 

 large in proportion to their size, and they are destitute of eye- 

 lids. The pupil, or the opening by which light penetrates in- 

 to the eye, varies much in form. In the greater part of the 

 species of which the eyes are vertical, anatomists remark a 

 singular disposition of the pupillary orifice, which presents the 

 fringes of the iris arranged in such a manner as to dilate or con- 

 tract, in order to weaken or augment the quantity of light which 

 enters the eye. In the Pleuronectes both eyes are on the same 

 side of the mesial line. 



The ear consists of a sac which represents the vestibule, in 

 which are suspended bones of a stony hardness, and of three semi- 

 circular membranous canals, situate rather in the cavity of the 

 cranium than in the substance of its walls, except among the 

 Chondropterygii. There is neither Eustachian tube nor tympa- 

 nal bones ; and the order Selachii have only an oval plate on a 



