THE MESENCEPHALON OR MIDBRAIN 613 



cannot be decided at present. Luciani, who formerly regarded the movements 

 as the effect of overstimulation on the peripheral stump of the peduncle left 

 by the wound, no longer expresses himself so positively. 



4. THE MESENCEPHALON OR MIDBRAIN 



The upper part of the midbrain is composed of the corpora quadrigemina 

 and the pineal body., which is now regarded as a remnant of an ancestral 

 eye ; the lower part consists of the crura cerebri. Both parts stand in intimate 

 relation to the visual organ, the former constituting a relay station in the 

 optic nerve, the latter containing nuclei for the most important intrinsic 

 and extrinsic muscles of the eye. In addition the midbrain, like all other 

 divisions of the brain except the cerebellum, is a conducting pathway. 



A. THE CORPORA QUADRIGEMINA 



When the optic lobes, which, in the lower vertebrates, correspond to the cor- 

 pora quadrigemina of the higher, are removed from fishes (Steiner), or from 

 the frog (Bechterew), the one prominent symptom is blindness. According to 

 Flourens the same thing is true in birds; but from more recent investigations 

 it appears that after unilateral destruction the power of vision in the opposite 

 eye is only reduced, not totally destroyed (Stefani). 



In dogs Bechterew found after destruction of one anterior body that the 

 homolateral halves of the two retinae became blind, although the defect in the 

 opposite eye was the more extensive. By more complete removal of both anterior 

 bodies almost total blindness was produced. The reaction of the pupil to light, 

 however, was very little affected, which is the case also after the same operation 

 in birds. 



Destruction of one posterior body produced disturbances to vision in the 

 median part of the opposite retina. 



In view of these observations, that in the mammals as well as in the lower 

 vertebrates the corpora quadrigemina are included in the optic tract, it is 

 the more remarkable that in men suffering from disease of the anterior bodies 

 no considerable derangement of vision has with certainty been established. 

 Suppression of one entire anterior quadrigeminal body occasions only a mod- 

 erate reduction of the visual power and leaves the color sense quite intact. 

 Likewise in the monkey, destruction of the anterior bodies produces no demon- 

 strable effect on vision (Bernheimer). 



Since now we know from anatomical discoveries that in both men and 

 monkeys the anterior body receives fibers from the optic nerve of the same 

 and of the opposite sides, and gives off fibers which can be followed to the visual 

 area of the cortex, we conclude, supposing the observations just mentioned 

 to be correct, that the bundle of fibers running between the visual area and 

 the anterior quadrigeminal body is of no direct importance for the act of 

 vision. There is evidence that this tract is concerned rather with certain 

 motor impulses discharged from the visual area, such for example as the 

 influence of visual impressions upon the movements of the eye and of the 

 body. 



