THE TERMINAL NERVE 929 



tex, with the encephalon. The optic nerve, therefore, corresponds more closely 

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



The oculomotor, trochlear, abducens and hypoglossal nerves are purely motor 

 nerves, and thus correspond only with the ventral roots of spinal nerves. The 

 spinal accessory 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 dis- 

 placed 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 funiculus of the cord on the dorsal side of the 

 ligamentum denticulatum, and from the upward prolongation of this funiculus. 



The cochlear and vestibular are nerves of special sense, and in some respects 

 both correspond closely with the dorsal root of a typical spinal nerve, and the gan- 

 glia of both represent spinal ganglia, but their distribution is limited to the mem- 

 branous labyrinth. 



The vagus and glosso-pharyngeal nerves contain both motor and sensory fibres, 

 but they differ from typical spinal nerves in that the motor fibres, in company 

 with the sensory, issue from the postero-lateral sulcus of the medulla, and they are 

 intimately intermingled, from their origin, with the sensory fibres, which latter 

 arise from ganglia interposed in the trunks of 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 

 attachments of the cranial nerves, and the groups of cells from which the fibres 

 spring, and about which they terminate, as their nuclei of origin or termination, 

 respectively. 



THE OLFACTORY NERVES 



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

 nerve cell-bodies situated in the olfactory region of the nasal mucous membrane, 

 in man, the olfactory region comprises the epithelium upon the superior third of 

 the nasal septum and that upon practically the whole of the superior nasal concha. 

 The area is relatively small as compared with that of other mammals and, as in 

 other mammals, is characterized by an increased thickness of the epithelium and a 

 yellowish brown colour in the fresh. The peripheral processes of the olfactory 

 cell-bodies (the olfactory ganglion) are short and extend only to the surface of the 

 olfactory epithelium. As the central processes pass upw^ard 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 upward, through the foramina in 

 the lamina cribrosa, into the anterior fossa of the cranium in tw^o rows, and after 

 piercing the dura mater, the arachnoid, and the pia mater, they enter the inferior 

 surface of the olfactory bulb. They contribute to 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 olfactor}" 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 derived from the pia 

 mater, from the subarachnoid tissue, and from the dura mater. Prolongations of the subarach- 

 noid space pass outward along the nerves for a short distance. 



Central connections. — The olfactory impulses are transmitted by way of the peripheral proc- 

 esses of the olfactory neurones through the cell-bodies and the olfactory nerve-fibres and 

 through the glomeruli to the mitral cells. Tlience they are carried by the central processes 

 (axones) of the mitral cells, which pass backward along each olfactory tract and its three olfac- 

 tory striae (see Rhinencephalon, p. 864). 



THE TERMINAL NERVE {Nervus Terminalis) 



In lower vertebrates and recently in those mammals whose sense of smeU is relatively much 

 more developed than in man, three nerves have been found concerned with the olfactory appara- 

 tus: — (1) The olfactory nerve proper whose fibres, as noted above, are the central processes of 



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