THE OPTIC NERVE 959 



cribrosa of the ethmoid bone directly into the olfactory bulb. The second nerve 

 is also a nerve of special sense. Its fibres form a very distinct bundle, similar in 

 appearance to an ordinary nerve, from which, however, it differs essentially, both 

 with regard to structure and development; for, unlike an ordinary nerve, its con- 

 nective tissue consists to a large extent of neuroglia instead of ordinary connective 

 tissue, and its component nerve-fibres are of much smaller calibre than those of an 

 ordinary nerve. It represents the location of the original optic stalk, a diverticulum 

 from the neural tube. The optic nerve, therefore, corresponds more closely with 

 an association tract of the central system than with an ordinary nerve. 



The third, fourth, sixth, and twelfth nerves are purely motor nerves, and thus 

 correspond only with the ventral roots of spinal nerves. The eleventh nerve is also 

 purely motor. Its fibres arise from the cells of the anterior horn of the spinal cord and 

 from a nucleus of the medulla which represents a displaced portion of that horn, but 

 they do not leave the surface of the spinal cord and brain in the usual situation of 

 ventral roots. On the contrary, they emerge in a series of rootlets from the lateral 

 column of the cord on the dorsal side of the ligamentum denticulatum, and from 

 the upward prolongation of the postero-lateral sulcus. 



The eighth or acoustic nerve is a nerve of special sense, and in some respects both 

 its parts correspond closely with the dorsal root of a typical spinal nerve, and the 

 ganglia of both its parts represent spinal ganglia, but its distribution is limited to 

 the sense organ. 



The ninth and tenth nerves contain both motor and sensory fibres, but they 

 differ from typical spinal nerves because the motor fibres, in company with the sensory, 

 issue from the postero-lateral sulcus of the medulla, and they are intimately inter- 

 mingled, from their origin, with the sensory fibres, which latter arise from ganglia 

 interposed in the nerves and otherwise correspond with the fibres of the dorsal root 

 of a typical spinal nerve. 



Superficial Attachments and Origins. It is customary to speak of the area 

 where the nerve-fibres leave or enter the brain substance as the superficial attachment 

 of the cranial nerves, and the group of cells from which the fibres spring as their cells of 

 origin. 



THE FIRST PAIR THE OLFACTORY NERVES 



The olfactory nerve-fibres are the central processes of the bipolar olfactory cells 

 situated in the olfactory region of the mucous membrane of the septal and outer walls 

 of the nasal fossa above the level of the lower border of the superior nasal concha. 

 As the fibres pass upwards from their cells of origin they form plexuses in the mucous 

 membrane, and from the upper parts of these plexuses, immediately below the 

 lamina cribrosa of the ethmoid, about twenty filaments issue on each side. These 

 filaments comprise the olfactory nerve. They are non-medullated. They pass up- 

 wards, through the foramina in the lamina cribrosa, into the anterior fossa of the 

 cranium in two rows, and after piercing the dura mater, the arachnoid, and the pia 

 mater, they enter the lower surface of the olfactory bulb. They pass through the 

 superficial stratum of nerve-fibres on the inferior surface of the olfactory bulb and 

 end in the glomeruli, which are formed by the terminal ramifications of the olfactory 

 nerve-fibres intermingled with the similar ramifications of the main dendrites of the 

 large mitral cells which lie in the deeper part of the grey substance of the olfactory bulb. 

 The olfactory nerve-fibres are grey fibres, since they do not possess medullary 

 sheaths, and they are bound together into nerves by connective-tissue sheaths de- 

 rived from the pia mater, from the subarachnoid tissue, and from the dura mater. 

 Prolongations of the subarachnoid space pass outwards along the nerves for a short 

 distance. 



Central Connections. The olfactory impulses are transmitted by way of the olfactory nerve- 

 fibres through the glomeruli to the mitral cells, and they are carried to the cerebrum by the 

 central processes (axones) of the mitral cells, which pass backwards along each olfactory tract 

 and its three striae olfactoriae (see Rhinencephalon, p. 844). 



THE SECOND PAIR THE OPTIC NERVES 



The fibres of the optic nerve are the central processes of the ganglion cells of 

 the retina. Within the bulbus oculi they converge to the optic papilla, where they are 



