630 Dynamic Theory. 



real visual roots are those in the geniculatum externum and pulvinar of 

 the optic thalamus, and the anterior brachium of the corpora quadri- 

 gemina. ( Ferrier.) 



The optic thalamus, like the other ganglia and the cerebrum, cannot 

 be normally stimulated by mechanical irritations. These simply injure 

 but do not cause normal reactions. The galvanic current alone is com- 

 petent to do this. When the optic thalamus is injured by mechanical 

 irritations, derangement is at once caused in the centers of muscular co- 

 ordination and equilibrium, and in the motor centers. A rabbit may 

 lose its cerebrum and its corpora striata and still be able to maintain 

 its balance, but if one-half of the optic thalamus be also removed, there 

 is a loss of sensibility on the opposite side of the body, and the animal 

 falls over toward that side. If both optic thalami be removed, the sen- 

 sibility of both sides will be destroj^ed, but after the shock of the lesion 

 is recovered from, some degree of co-ordination and equilibration will 

 remain through the influence of the muscular sense, the medulla and 

 cerebellum. If, without removing the cerebrum, an incision be made 

 in one of the optic thalami, the animal keeps turning to one side in a 

 circular manner. According to Schiff, the destruction of the three an- 

 terior fourths of this organ in rabbits, causes the movement towards 

 the injured side, and the lesion of the posterior fourth towards the op- 

 posite side. Extensive disease of the optic thalamus or the corpus stri- 

 atum of one side, produces hemiplegia, or paralysis of both sensation 

 and motion on the other side. The same effect is produced usually by 

 an apoplectic effusion of blood into the substance of either of these 

 ganglia. ( Carpenter. ) 



In the higher vertebrates, the optic thalami are condensing organs for 

 all sense stimulations, which they forward to the cells of the corpus 

 striatum and the cerebrum, and they therefore receive stimulation from 

 all the external senses; sight, hearing, smell, taste, touch, and the mus- 

 cular sense. There are no optic thalami in the lowest of the vertebrates, 

 and but little development of it in any of the fishes. The function they 

 perform for the higher mammals is of a far simpler nature in the fishes, 

 and such as it is, it is performed, we may infer, by the other sensory 

 ganglia. The corpora quadrigemina, which are largely developed in 

 these animals, receive the optic stimuli and distribute them to the cor- 

 pus striatum, the cerebellum and the cerebrum; the route to the latter be- 

 ing by way of the posterior edge of the crura cerebri, the part upon which, 

 in the other vertebrates, the optic thalamus is developed. This tract is 

 also the route to the cerebrum for all the other sense stimuli, unless 

 the sense of smell be an exception, which, in the fishes, it probably is. 



The pituitary and pineal glands, which, with their appendages, are 

 parts of the thalamencephalon, are already developed in the fishes. 



