646 THE NERVOUS SYSTEM 



atrophy. It has been further shown that in a newly-born animal the 

 removal of such a region is followed by imperfect development of the parts 

 in question. 



If one optic nerve be divided, blindness of the corresponding eye results; 

 but if one optic tract be divided there is a half blindness in both eyes, which is 

 called hemianopsia, or hemiopia, right or left, according as the right or left 

 field of vision is cut off. It is also evident that the occipital lobe, figures 403, 

 406, and particularly the cuneus, is concerned as a visual center. Not only 

 is the occipital lobe connected with the optic nerves, as we have seen, but 

 the removal of the right occipital lobe in an animal (monkey) is followed by 

 bilateral hemiopia of the right retinae. Removal of the left occipital lobe is 

 followed by left bilateral hemiopia. Removal of both occipital lobes is fol- 

 lowed by total blindness. Clinical observations give convincing proof that 

 in man also the occipital lobes are the cortical centers of vision. Disturb- 

 ance of the optic lobes or tumors involving the optic tracts, figure 404, lead to 

 disturbance and blindness in the retinal field. 



Olfactory Center in the Cortex. The olfactory nerve differs from 

 the other cranial nerves. In reality it is a representative of the olfactory lobes 

 of other animals, which are part of the cerebrum. The olfactory lobe origi- 

 nates as an offshoot from the cerebral vesicle, the front part of which is de- 

 veloped into the bulb of the olfactory nerve, while the back part forms its 

 peduncle. The nerve, the cavity of which in man is filled up in the fully de- 

 veloped condition with neurogliar substance, lies upon the cribriform plate 

 of the ethmoid bone, and is contained in a groove on the under surface of 

 the frontal lobe. On examination of the lobe it is found to be thus made up: 

 Beneath the neurogliar layer is a layer of longitudinal fibers and a few 

 nerve cells, next to this is a layer of small cells, nuclear layer, fibers from the 

 layer of nerve fibers passing through it. 



The nuclear layer is also separated into groups of cells by an interlacing 

 of the fibers. The next layer is thick and is composed of neuroglia and nerve 

 fibers, some of which are medullated, as well as of cells more or less pyramidal 

 in shape. Below this layer is the layer of olfactory glomeruli. These glomer- 

 uli are small synapses of olfactory fibers. The larger also includes small 

 cells and granular matter. A further description of the anatomy of these 

 parts is given later, page 656. 



Fibers of the olfactory nerve proper are found below this layer, and pass 

 through the cribriform plate to be distributed to the olfactory mucous mem- 

 brane. They arise from cells in the olfactory mucous membrane, and end in 

 the glomeruli. The peduncle of the nerve or the olfactory tract, as it is some- 

 times called, is made up of longitudinal fibers originating in the bulb, with 

 neuroglia and some nerve cells. 



The fibers of the olfactory tract have been traced into the nucleus amyg- 

 dalae and its juncture with the hippocampal gyrus in the temporal lobe, 



