The Nervous System 1 1 3 



drical and inelastic. In a few fishes (head-fish, trunk-fish) in 

 which the posterior part of the body is shortened or degener- 

 ate, the spinal cord is much shortened, and replaced behind by 

 a structure called cauda equina. In the head-fish it has shrunk 

 into "a short and conical appendage to the brain." In the 

 Cyclostomes and chimaera the spinal cord is elastic and more 

 or less flattened or band-like, at least posteriorly. 



The Nerves. The nerves of the fish correspond in general 

 in place and function with those of the higher animals. 

 They are, however, fewer in number, both large nerve-trunks 

 and smaller nerves being less developed than in higher 

 forms. 



The olfactory nerves, or first pair, extend through the ethnoid 

 bone to the nasal cavity, which is typically a blind sac with two 

 roundish openings, but is subject to many variations. The optic 

 nerves, or second pair, extend from the eye to the base of the 

 optic lobes. In Cyclostomes these nerves run from each eye to 

 the lobe of its own side. In the bony fishes, or Teleostei, each 

 runs from the eye to the lobe of the opposite side. In the sharks, 

 rays, chimaeras, and Ganoids the two optic nerves are joined in 

 a chiasma as in the higher vertebrates. 



Other nerves arising in the brain are the third pair, or ner- 

 vus oculorum motorius, and the fourth pair, nervus trochlearis, 

 both of which supply the muscles of the eye. The fifth pair, 

 nervus trigeminus, and the seventh pair, nervus facialis, arise 

 from the medulla oblongata and are very close together. Their 

 various branches, sensory and motor, ramify among the mus- 

 cles and sensory areas of the head. The sixth pair, nervus ab- 

 ducens, passes also to muscles of the eye, and in sharks to the 

 nictitating membrane or third eyelid. 



The eighth pair, nervus acousticus, leads to the ear. The 

 ninth pair, glosso-pharyngeal, passes to the tongue and pharynx, 

 and forms a ganglion connected with the sympathetic system. 

 The tenth pair, nervus vagus, or pneumogastric nerve, arises from 

 strong roots in the copus restiforme and the lower part of the 

 medulla oblongata. Its nerves, motor and sensory, reach the 

 muscles of the gill-cavity, heart, stomach, and air-bladder, as 

 well as the muscular system and the skin. In fishes covered 

 with bony plates the skin may be nearly or quite without sen- 



