52G THE CRANIAL NERVES. 



On the preceding- two pages. Fig. ."31 is introduced in illustration of the general 

 view of the distribution above given. In this figure the cranium and orbit have 

 been opened up to the depth of the several foramina through which the nerves 

 pass. The greater part of the lower jaw has also been removed on the left side, 

 and the tongue, pharj-nx, and larynx are partially in view-. The occipital bone 

 has been divided by an incision passmg down from the occipital tuberosity and 

 through the condyle to the left of the foramen magnum. The cervical vertel^raa 

 have been divided to the left of the middle, and the sheath of the spinal cord 

 opened so as to expose the roots of the cervical nerves. 



OLFACTORY NERVE. 



The first or olfactory nerve, as it is usually termed, or tract, as it has 

 been more correctly designated, is the special nerve of the sense of 

 smell. It lies on the under (orbital) surface of the frontal lobe of the 

 cerebrum, lodged in a sulcus (olfactory sulcus) to the outer side of and 

 parallel to the longitudinal median fissure. In front the tract swells into 

 an oval enlargement, the olfactorij bulb, which lies in the same sulcus, and 

 from which small nerves descend, through the cribriform plate, into the 

 nose. The olfactory tract and bulb contain a considerable quantity of 

 grey matter, and, in respect of structure, connections, and development, 

 are to be regarded rather as constituent parts of the cerebrum than as 

 a, true nerve, — the true peripheral olfactory nerves taking origin from 

 the lower surface of the bulb. In accordance, however, with the most 

 common practice in anatomical Avorks, the nerve will be here described 

 as arising at the commencement of the tract in the brain. A fuller 

 description of the tract and bulb will be found in the account of the 

 cerebrum (Vol. ii., pp. 536 and 562). 



Surface attachment. — At its posterior extremity the olfactory tract 

 widens out and is attached to the under surface of the frontal lobe, in 

 front of the anterior perforated space, by means of three roots, named 

 external, middle and internal, which pass in different directions. 



The external or Jong root consists of a band of medullary fibres, which 

 passes, in the form of a white streak, outwards and backwards along 

 the anterior margin of the perforated space, towards the posterior border 

 of the Sylvian fissure, where it disappears. 



The middle or grey root (tuber olfactorium) is of a pyramidal shape, 

 and consists of grey matter on the surface, which is prolonged from that 

 of the adjacent part of the anterior lobe and of the perforated space. 

 "Within it there are white fibres, which are sometimes described as alone 

 constituting the middle root. 



The internal root (short root, Scarpa), which cannot always be demon- 

 strated, is composed of white fibres which may be traced from the inner 

 and posterior part of the anterior lobe. 



Beep origin. — This is still doubtful. The outer root has been 

 traced by different observers to the island of Eeil, the optic thalamus 

 (Valentin), and to a nucleus in the substance of the temporo-sphenoidal 

 lobe in front of the anterior extremity of the hippocampus (Rolando, 

 Luys, Foville. This agrees also with Terrier's experimental localisation 

 of the sense of smell in the monkey). The fibres of the inner root have 

 been thought to be connected with the anterior extremity of the gyrus 

 fornicatus, or to cross over to the opposite hemisphere. The fibres of 

 the middle root have been said liy some to join those of the inner root, 

 by others to be connected with the corpus striatmn. 



