50 



in all fishes, lodged between the hemispheres and optic tubercles, 

 but so small in the osseous tribes that its existence has been ques- 

 tioned. These several lobes are covered with a layer of cineritious 

 substance, and closely invested by a delicate layer of pia mater; 

 outside of which is the soft, gelatinous, cellular arachnoid tunic, and 

 all are surrounded by an envelope of dura mater. 



Nerves. — The olfactory nerves are white and fibrous ; they are 

 very large in the rays and sharks, and in many instances form a 

 ganglion before their termination ; as may be seen in the carp. 

 The optic nerves are developed in proportion to the size of the optic 

 tubercles and eyes ; hence they are large in the carp, and slender in 

 the eel. In the osseous fishes they generally cross without any in- 

 termingling of fibres; in the plagiostome fishes their fibres are 

 blended at the commissure as in the mammalia; and in the skate, 

 the right nerve goes through a fissure in the left. The third, fourth, 

 and sixth nerves are developed in proportion to the size of the 

 muscles they supply. The fifth nerve presents a greater size, and 

 gives off more branches in this class than in any other of the verte- 

 brata. The ophthalmic, superior, and inferior maxillary are dis- 

 tributed to the face, palate and lower jaw. In the rays, and many- 

 other fishes, it sends a branch to the ear, and in the torpedo, to the 

 electrical organs. These latter are composed of a series of mem- 

 branous cells occupied by a gelatino-albuminous substance, per- 

 forming the office of a Leyden jar or electrical battery. They lie 

 in the torpedo on the upper surface of the lateral fin. In the elec- 

 tric eel, on the posterior part of the abdomen ; and in the silurus 

 electricus they are situate between the muscles and skin over the 

 entire body. In the last named fish the nerves appear to be derived 

 from the pneumo gastric. Both portions of the seventh pair are 

 small and distributed without any peculiarity. The pneumo-gas- 

 tric arises from the side of the medulla oblongata, behind the fifth 

 pair ; it forms a large ganglion below its origin, from which 

 branches proceed to the branchiae, the oesophagus, the stomach, and 

 rudimentary lungs. This nerve gives off also a branch to the 

 tongue analogous to the glossopharyngeal, and one to the lateral 

 part of the body analogous to the spinal accessary, both of which 

 are rudimental in fish. The ninth pair is wholly absent and its 

 place supplied by a branch from the fifth. The distribution of the 

 spinal nerves is very simple, and their development is always pro- 

 portioned to the size of the fins ; the sympathetic is very slender, 

 and its ganglions small in fishes. It is most developed in the pla- 

 giostome chondropterygii, and least so in the cyclostome species ; it 

 receives filaments from the spinal nerves, and its meshes accompany 

 the arterial trunks in their distribution on the digestive, respiratory, 

 and generative organs. 



AMPHIBIA. 



The condition of the brain, the medulla oblongata, and the spinal 

 chord are nearly the same in the perennibranchiate amphibia, and 



