The _olfactory nerves_, or first pair, extend through the ethmoid 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, chimæras, 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 _nervus 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 muscles and sensory areas of the head. The sixth pair, _nervus abducens_, 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 corpus 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 sensory nerves. The eleventh pair, _nervus accessorius_, and twelfth pair, _nervus hypoglossus_, are wanting in fishes. The spinal nerves are subject to some special modifications, but in the main correspond to similar structures in higher vertebrates. The anterior root of each nerve is without ganglionic enlargement and contains only motor elements. The posterior or dorsal root is sensory only and widens into a ganglionic swelling near the base. A sympathetic system corresponding to that in the higher vertebrates is found in all the Teleostei, or bony fishes, and in the body of sharks and rays in which it is not extended to the head. FOOTNOTES: [6] The thalamencephalon or the interbrain is a name given to the region of the optic thalami, between the bases of the optic lobes and cerebrum. [7] Fishes Recent and Fossil, p. 55. CHAPTER VIII THE ORGANS OF SENSE =The Organs of Smell.=--The sense-organs of the fish correspond in general to those of the higher vertebrates. The sense of taste is, however, feeble or wanting, and that of hearing is muffled and without power of acute discrimination, if indeed it exists at all. According to Dr. Kingsley (Vert. Zool., p. 75), "recent experiments tend to show that in fishes the ears are without auditory functions and are solely organs of equilibration."