224 NERVOUS SYSTEM. 



having six of each. The posterior extremity of the 

 chord is sometimes sensibly enlarged where nerves pro- 

 ceed to the muscles of a large caudal fin, and in abdo- 

 minal fishes an enlargement is observed, corresponding 

 with the ventral fins. In some fishes with a great de- 

 velopment of the head and anterior portion of the trunk, 

 as the frog-fish^ and the tetrodon, the spinal chord passes 

 but a short way through the vertebral canal, and a long 

 cauda equina extends backwards, as in the human body. 

 The symmetrical nerves arise by double roots from the 

 two grooves on each side of the spinal chord, the motor 

 nerves which commence more towards the tail than the 

 sensitive, originate from the anterior lateral groove, and 

 the sensitive nerves, provided each with a ganglion be- 

 yond the vertebral canal, originate from the posterior 

 groove. These unite, as in the invertebrated classes, to 

 form mixed, moto-sensitive nerves, they give sensibility 

 and motility to the organs of animal life, and they send 

 filaments to the sympathetics each vertebra being analo- 

 gous to a segment of the trunk, and each pair of sym- 

 metrical nerves originating from the brain of that segment. 

 The nervous O3sophageal ring of the invertebrata is still 

 perceptible in the wide opening between the lateral halves of 

 the medulla oblongata of the lampreys. The great fasci- 

 culi composing the cerebral masses the corpora py- 

 ramidalia, olivaria and restiformia are already obvious 

 in the large medulla oblongata of fishes ; but the 

 crossing fasciculi of the corpora pyramidalia are slightly 

 marked, they become apparent and numerous as we ascend 

 through higher classes. The medulla oblongata, the apparent 

 origin of most of the cranial nerves, is here large and lobed, 

 and often nearly as broad as the cerebral organs before it in 

 the cranium ; it is deeply marked above by a calamus scrip- 

 torius, at the bottom of the fourth ventricle, which is situate 

 between it and the single median lobe composing the cere- 

 bellum. 



Anterior to the medulla oblongata and cerebellum, there 

 are generally in osseous fishes three pairs of rounded lobes 

 placed in front of each other along the floor of the cranium, 

 and occupying but a small portion of that capacious cavity, 

 as seen in the brain of the conger-eel, muraena conger 



