NERVES. 105 



of the ophthalmic division of the Trigeminus. In Pctromyzon 

 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 Ncrvus 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 ahduccns (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 mem- 

 brane of Sharks. 



The Ncrvus trigeminus {fifth pair) and the Ncrvus facialis 

 {seventh pair) have their origins close together, and enter into 

 intimate connection with each other. In the Chondroptery- 

 gians and most Teleostei the number of their roots is four, in 

 the Sturgeons five, and in a few Teleostei three. When there 

 are four, the first issues immediately below the cerebellum 

 from the side of the Medulla oblongata ; it contains motory 

 and sensory elements for the maxillary and suspensorial 

 muscles, and belongs exclusively to the trigeminal nerve. The 

 second root, which generally becomes free a little above the 

 first, supplies especially the elements for the Eamus palatinus, 

 which sometimes unites with parts of the Trigeminal, some- 

 times with the Facial nerve. The third root, if present, is very 

 small, and issues immediately in front of the acustic nerve, 

 and supplies part of the motor elements of the facial nerve. 

 The fourth root is much stronger, sometimes double, and its 



