FISHES. 45 



fishes ^ The fifth and the tenth pairs of nerves (the nerviis tri- 

 geminus and n. vagus) are remarkably developed. The facial nerve, 

 Avhicli by many writers is denied to fishes, appears to be represented 

 in bony fishes by the oimrcular hrancJi of the fifth pair. Of the 

 three pairs of nerves of special sense that for the eyes is tlie most 

 developed in the greatest number of fishes. The optic nerves 

 arise from the hollow lobes that compose the middle brain, and are 

 at their origin connected by transverse bundles. In the Plagio- 

 stomes there is a chiasma or crossing of the fibres, but in the bony 

 fishes the two nerves themselves lie crosswise upon one another ; 

 so that the nerve which springs from the right side runs to the left 

 eye, and that from the left side to the right eye. Here, where the 

 nerves cross one another, they are not connected by any nervous 

 tissue, so that they may be separated and thrown apart without 

 artificial division. The olfactory nerve, when there is no ganglionic 

 swelling in front of the hemisphere of the cerebrum, has such a 

 swelling at its extremity, immediately before it enters the olfactory 

 organ, as in Gadus, Silurus, most Cyprini, and the Plagiosfomes. 

 The auditory nerve arises, close to the fifth pair, from the medulla 

 oblongata. The n. vagus arises by two roots from the medulla oblon- 

 gata ; the posterior root is the most conspicuous, and the medulla ob- 

 longata sometimes presents here a considerable swelling (lobus nervi 

 vagi") as in Cyprinus. This nerve mostly surpasses the trigeminus 

 in thickness ; its branches run especially to the branchial arches, but 

 besides this to the oesophagus, the stomach, the heart, the swim- 

 ming-bladder, and in Torpedo and Malapterurus electricus to the 

 electric organ. Moreover from the ii. vagus a nerve arises, which 

 runs longitudinally amongst the large lateral muscles, sometimes 

 deeper, in other instances immediately beneath the skin, and which, 

 in those fishes where it lies deeper, gives off a superficial branch 

 which runs longitudinally under the Unea lateralis. This nervus 



1 In the Myxinoidce the nerves of motion of the eye are wanting, accordino- to 

 Mueller. Compare also on the peripheral nervous system of fishes, besides the works 

 already cited, especially Buechner Memoire sur le syst. nerveux clu Barbeau, Memoires 

 de la Soc. d'Hist. nat. de Strasbourg, ii. 1835, and H. Stannius Das peripherische 

 Nervensystem der Fische. Mit 5 Steintafeln. Rostock, 1849, 4to. 



2 See a figure of it, given by E. H. Weber, in Meckel's Archiv, 1827, Tab. iv. 

 fig. 26. 



