576 Dynamic Theory. / 



varolii. The optic lobes are formed upon the posterior parts of the me- 

 lullary stem, and their function is chiefly, though perhaps not exclu- 

 sively, the condensation of the stimuli of sight. A part of the roots 

 >f the optic nerve are found in the optic lobes, and part are to be traced 

 in the hypoaria. The optic lobes vary greatly in size. In the nearly 

 blind cave fish, Amblyopsis speleus, both these lobes and the optic 

 nerves are very small, and less in size than either the cerebellum or 

 cerebrum. They are likewise smaller than these organs, relatively, in 

 the chimera and the shark. They exist in the myxine, a blind parasite, 

 from which circumstance Owen infers that their function relates to 

 something more than merely sight stimuli. But in this fish perhaps 

 they are rudimentary. In most fishes they are larger than the cerebral 

 lobes, in a few, as the polypterus and lepidosiren, they are larger than 

 the cerebellum but smaller than the cerebrum. In the Sturgeon, Pol- 

 ypterus, Lepidosiren, Amblyopsis and Loach (Cobitis), the optic lobes 

 are scarcely separated from each other, but run together. They are 

 distinct in most osseous fishes, but are connected together by a commis* 



FIG. 287. Brain of Amblyopsis, (" obscure-eyed") cave 

 fish. 



c. Cerebellum extending over the optic lobes. 



o. Optic lobes. P. Cerebrum. 



to. Pineal gland. R. Olfactory lobes. 



m. Medulla oblongata. 1. Olfactory nerves. 



2. Optic nerves greatly reduced by disuse- 



( No pons varolii in fishes, and no lateral lobes to the 

 cerebellum. 



sure, generally the anterior commissure pass- 

 ing across from one to the other in front of 

 the entrance to the third ventricle. These 

 lobes are generally hollow, or contain ventri- 

 cles, one in each. On the floor of each of 

 these there are usually one or two bulbs, or 

 FIG. 287. tubercles, called optic tubercles. In the Cod, 



Salmon, Pike and Perch there are four of these, two in each lobe ; in 

 the Carp and Herring there is one in each lobe. The Sharks and Rays, 

 the Sturgeon, Polypterus and Lepidosiren do not possess these tubercles. 

 Those which do have the tubercles also possess another process inside 

 the ventricle,"" called the " torus semicircularis. " This is a sort of an 

 elongated tubercle, which is attached to the floor of the ventricle in ad- 

 vance of the "optic tubercles, " and arches backward over them to a 

 greater or less extent. They are large in the Carp. Neither these ven- 

 tricular tubercles nor the torus are found in any animals besides fishes. 

 According to Owen, these tori are not to be regarded as homologous 

 with either the optic thalamus or corpus striatum. The external shell 

 of the optic lobes is composed of white and gray matter. These are 

 shown blended upon the surface. 



The restiform columns of the medulla oblongata pass into the base 



