THE CRANIAL NERVES 645 



The center itself contains the following subdivisiofis: (a) The lateral 

 olfactory nucleus which extends backward into the tip of the temporal 

 lobe of the cerebrum as far as the point of contact between the ventro- 

 lateral extremities of the hippocampus and hippocampal gyrus, 

 (b) the medial olfactory nucleus into which the medial olfactory striae 

 are discharged, and (c) the intermediate olfactory nucleus in the anterior 

 perforated substance in which the intermediate olfactory striae termin- 

 ate. These nuclei are intimately connected with other cerebral centers 

 and diverse motor paths, thereby enabling the sensory impressions 

 of smell to become associated with other sensations as well as with 

 the different motor mechanisms. This close correlation permits these 

 nuclei to play the part of reflex centers, in which the olfactory impulses 

 are transferred to efferent paths and to the motor end-organs. In 

 man, these olfactory reflex centers are dominated by a psychic or 

 cortical center which, as will be shown later, occupies the hippocampal 

 convolution, especially its distal end, the uncus. Different association 

 paths connect this area with other cortical centers. 



2. The optic nerve, or nerve of sight, conveys the impulses from 

 the retina to the thalamus, where they are transferred onward to the 

 center for vision in the occipital region of the cerebral cortex. The 

 , essential receptive element of the eye is the retina which forms the 

 'innermost coat of this sense-organ and contains neurons of the fol- 

 lowing four types: (a) The rods and cones, (6) the bipolar cells, (c) 

 the ganglion cells, and (d) the horizontally arranged association neu- 

 rons. The fibers of the optic nerve take their origin from the ganglion 

 cells, but this does not mean that these elements constitute neurons 

 [of the first order. In fact, as the real receptors of the retina are the 

 [rods and cones, these elements should be regarded as forming the 

 peurons of the first order of the optic path. Their impulses are 

 [transmitted across the external molecular layer to neurons of the sec- 

 [ond order, the cell bodies of which are situated in the internal granular 

 flayer. These data tend to show that the ganglion cells of the retina 

 [are already neurons of the third order which then leave the eye through 

 :he optic papilla to form the optic nerve proper. 



Having reached the optic chiasma at the ventral aspect of the 



jrebrum, these fibers enter into a decussation which carries them either 



part or as a whole to the opposite side of the brain. A complete 



jrossing is effected in fishes, amphibians, reptiles and most birds, and a 



)artial one in man and the mammals, namely, in those animals in 



rhich the visual fields overlap and which possess stereoscopic vision. 



'here is, however, no evidence at hand to show that the crossing in 



ihe latter is absolutely symmetrical, because the number of fibers 



remaining on the same side seems to become the greater, the higher 



the rank of the animal in the scale of the Animal Kingdom. In man, 



lowever, the fovea centralis or yellow spot seems to be innervated 



bilaterally, i.e., the fibers emerging from this area pass to both visual 



centers. This crossing carries the fibers from the inner halves of the 



