NERVOUS SYSTEM IN THE VERTEBRATA. 49 



occupies a dorsal situation, and is protected by an osseous sheath. 

 In the lowest orders of fishes, as the lamprey, and the gastrobranchus, 

 we perceive a repetition of the two nervous columns extending 

 along the back as observed in the worm. This simple condition 

 resembles the embryo state of this system in the highest grades of 

 the vertebrata, previous to the development of their extremities. 

 With kw exceptions the spinal chord extends the whole length of 

 the vertebral column ; whence, from the great number of vertebrae, 

 it attains a very remarkable length. In some, however, as the 

 lophius piscatorius, it is stated to be very short, forming a kind of 

 Cauda equina as in man. It usually terminates in a single thread, 

 presenting several enlargements throughout its tract, which corres- 

 pond very accurately with the number, magnitude, and situation of 

 the extremities. For instance, when the anterior members are very 

 large, as in rays and flying-fishes, the anterior enlargements are 

 proportionally developed ; and where a large caudal fin is to be 

 supplied, the chord presents a sensible enlargement posteriorly 

 where the nerves join it. The spinal marrow, here, as in the 

 human foetus, usually contains a canal of considerable size, and is 

 distinguished by an anterior, a posterior, and two lateral grooves. 

 From the latter, the nerves arise by two roots, the posterior or sen- 

 sitive root having a ganglion on it, receives the anterior root im- 

 mediately external to the canal. 



The brain m fishes does not fill the cavity of the cranium, a 

 considerable portion of it being occupied by the soft cellular tissue 

 of the arachnoid. The medulla oblongata is of great length, lobed, 

 and deeply grooved above by the calamus scriptorius. 



Tn most fishes, the optic lobes are larger than the hemispheres, 

 they are hollow and communicate freely with each other and with 

 the fourth ventricle. The earlier they are examined the larger they 

 are found to be ; their development is proportionate to that of the 

 optic nerves and eyes, and inversely to that of the cerebrum and 

 cerebellum. The cerebral hemispheres are small in the osseous, 

 apodal, and cyclostome fishes, and in the plagiostome species they are 

 larger than the optic tubercles. In the osseous fishes they resemble 

 the embryo condition of the human brain in being destitute of ven- 

 tricles, and having no convolutions on the surface. In sharks and 

 rays the hemispheres attain a large size, present irregularities on 

 the surface, and ventricles in their interior. In front of the hemi- 

 spheres are placed the olfactory tubercles, elongated transversely, and 

 exceeding in magnitude the hemispheres themselves; subject, how- 

 ever, to considerable variations, regarding form, size, and situation. 



The cerebellum is scarcely to be recognised in many of the 

 cyclostome fishes. When present it appears as a transverse band, 

 rising vertically in the osseous fishes, and forming a small vermiform 

 median lobe, slightly laminated in the plagiostome fishes. In sharks 

 and rays not only does this median lobe attain considerable magni- 

 tude, but small hemispheres are developed laterally, corresponding 

 in size to that of the carpora restiformia. The pineal gland is found 

 8 — e evers 4 



