The _optic_ nerves (_second pair_) vary in size, their strength corresponding to the size of the eye; they take their origin from the _lobi optici_, the development of which again is proportionate to that of the nerves. The mutual relation of the two nerves immediately after their origin is very characteristic of the sub-classes of fishes. In the _Cyclostomes_ they have no further connection with each other, each going to the eye of its own side.[11] In the _Teleostei_ they simply cross each other (_decussate_), so that the one starting from the right half of the brain goes to the left eye and _vice versa_. Finally, in _Palæichthyes_ the two nerves are fused together, immediately after their origin, into a _chiasma_. The nerve is cylindrical for some portion of its course, but in most fishes gradually changes this form into that of a plaited band, which is capable of separation and expansion. It enters the bulbus generally behind and above its axis. The foramen through which it leaves the skull of Teleostei is generally in a membranous portion of its anterior wall, or, where ossification has taken place, in the orbito-sphenoid. B. _Nerves proper taking their origin from the brain_ (Figs. 41–45). The _Nervus oculorum motorius_ (_third pair_) takes its origin from the Pedunculus cerebri, close behind the lobi inferiores; it escapes through the orbito-sphenoid, or the membrane replacing it, and is distributed to the musculi rectus superior, rectus internus, obliquus inferior, and rectus inferior. Its size corresponds to the development of the muscles of the eye. Consequently it is absent in the blind _Amblyopsis_, and the Myxinoids. In _Lepidosiren_ the nerves supplying the muscles of the eye have no independent origin, but are part of the ophthalmic division of the Trigeminus. In _Petromyzon_ these muscles are supplied partly from the Trigeminus, partly by a nerve representing the Oculo-motor and Trochlearis, which are fused into a common trunk. The _Nervus trochlearis_ (_fourth pair_), if present with an independent origin, is always thin, taking its origin on the upper surface of the brain from the groove between lobus opticus and cerebellum; it goes to the Musculus obliquus superior of the eye. C. _Nerves taking their origin from the Medulla oblongata_ (Figs. 41–45). The _Nervus abducens_ (_sixth pair_) issues on the lower surface of the brain, taking its origin from the anterior pyramids of the Medulla oblongata, and supplies the Musculus rectus externus of the eye, and the muscle of the nictitating membrane of Sharks.