930 THE NERVOUS SYSTEM 



the nerve cell-bodies situated in the epithelium of the olfactory region of the nasal mucosa, and 

 which terminate in the olfactory bulb ; (2) The vomero-nasal nerve, whose fibres are the central 

 processes of nerve ceU-bodies situated in the epithelium of the vomero-nasal (Jacobson's) organ 

 and which pass caudalward in the submucosa and upward to join the filaments of the olfactory 

 nerve proper and which, in the dog, cat, rabbit, rat, etc., terminate in the accessory olfactory 

 bulb — a small protuberance possessed by these animals on the postero-median aspect of the 

 olfactory bulb proper; (3) The terminal nerve, a small plexiform nerve, which unlike the other 

 two, is ganglionated. 



In man, the vomero-nasal (Jacobson's) organ is rudimentary after birth and, therefore, the 

 vomero-nasal nerve is not present, the only fibres for the vomero-nasal region being those of gen- 

 eral sensibility from the trigeminus and sympathetic fibres common to the epithelium of the 

 entire nasal fossa. 



The terminal nerve has been recently described as present in the human foetus and it is men- 

 tioned here because of the expressed belief that it is present in the adult. From the observations 

 recorded for human and rabbit foetuses and the adult dog and cat, the following description may 

 be given : It is variably plexiform throughout its course. Its peripheral twigs are distributed 

 to the mucosa of the nasal septum, some to the mucosa joining the olfactory region while other 

 and larger twigs extend further forward and are distributed to mucosa of the vomero-nasal organ, 

 accompanying and sharing in the distribution of the vomero-nasal nerve when this is present. 

 Its central connections are in the form of two or three small roots which pass through the cribri- 

 form plate of the ethmoid bone in company with and mesial to the vomero-nasal nerve and then, 

 still plexiform, extend caudalward over the infero-mesial aspect of the olfactory bulb and upon 

 the olfactorj^ peduncle or stalk (olfactory tract) beyond, a root often extending to near the 

 lamina terminalis and optic chiasma. The roots disappear in the mesial and infero-mesial 

 aspect of the frontal portion of the brain at different localities caudal to the olfactory bulb and 

 usually near the olfactory peduncle, but often one may disappear in the region corresponding to 

 the anterior perforated substance of the adult human brain. 



Numerous small groups of ganglion cells are found interposed along both the peripheral and 

 intracranial course of the terminal nerve. A group, larger in size than the others and situated in 

 the intracranial course of the nerv^e, is called the ganglion terminale. The fibres of the nerve are 

 non-medullated. Both the ganglion cells and the fibres of the nerve are described as having 

 more the appearances characteristic of sympathetic neurones than of cranio-spinal. On the 

 other hand, our conceptions of sympathetic neurones do not permit of their terminating within 

 the central system except for the innervation of its bloodvessels. It may result that, instead of 

 being an independent nerve as now claimed, the nervus terminalis is a part of the forward exten- 

 sion of the cephalic sympathetic, the larger ganglia and plexuses of which latter are well known, 

 and that its neurones receive and convey impulses to the gland cells of the nasal mucosa and to 

 the muscle of the blood-vessels of the mucosa and those supplying the infero-mesial part of the 

 frontal end of the cerebrum. 



THE OPTIC NERVES 



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

 retina. Within the ocular bulb they converge to the optic papilla, where they 

 are accumulated into a rounded bundle, the optic nerve. The nerve thus formed 

 pierces the chorioid and the sclerotic coats, and, at the back of the bulb, enters the 

 orbital fat, in which it passes backward and medialward to the optic foramen. 

 After traversing the foramen it enters the middle fossa of the cranium, and anas- 

 tomoses with its fellow from the opposite side, forming the optic chiasma. It 

 may, therefore, for descriptive purposes, be divided into four portions — the 

 intra-ocular, the intra-orbital, the intra-osseous, and the intra-cranial. The 

 total length of the nerve varies from forty-five to fifty millimetres. 



The intra-ocular part is rather less than one millimetre in length. It passes 

 backward from the optic papilla through the chorioid and through the sclerotic 

 coats of the bulb. As it passes through the latter coat of the bulb in many sep- 

 arate bundles, the area it traverses has a cribriform appearance when the nerve 

 is removed, and consequently is known as the lamina crihrosa sclerce. 



The intra-orbital part of the nerve emerges from the sclerotic about three milli- 

 metres below and to the median side of the posterior pole of the bulbus, and it is 

 about thirty millimetres long. It passes backward and medialward, surrounded 

 by the posterior part of the fascia bulbi (Tenon's capsule) and by the orbital fat, 

 to the optic foramen. 



As it runs 1)ackward in the orbit it is in relation above with the naso-ciliary (nasal) nerve and 

 the ophthalmic artery which i)ass obliquely from beliindand laterally, forward and medialward 

 across the junction of its posterior and middle thirds, and also in relation with the superior oph- 

 thalmic vein, tlio .superior rectus muscle, and the upper branch of the oculo-motor nerve. Below 

 it arc the inferior rectus muscle, and the inferior division of tlie oculo-motor nerve. To its lateral 

 side, near the posterior part of the orbit, are the ophthalmic artery, the ciliary ganglion, the 

 abducens nerve, and the external rectu.s muscle. The anterior two-thirds of this portion of the 

 optic nerve are surrounded l>y the ciliary arteries and the ciliary nerves and it is penetrated on.its 



