584 THE CRANIAL NERVES. 



agrees closely in structure with the cerebral substance. It swells into an oval 

 enlargement, the olfactory bulb, in front, which also contains much grey 

 matter, and from this part small soft nerves descend through the cribriform 

 plate into the nose. When traced backwards, it is found to be spread out 

 and attached behind to the under surface of the anterior lobe by means of 

 three roots, named external, middle, and internal, which pass in different 

 directions. The bulbous part is therefore rather to be regarded as an 

 olfactory lobe of the cerebrum than as a part of a true nerve, while the 

 white part prolonged backwards into the brain, together with its so-called 

 roots, may be termed the olfactory tract. 



The external or long 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 may be followed into the substance of the cere- 

 brum. Its further connections are doubtful, but it has been stated that its 

 fibres have been traced to the following parts, viz., the convolutions of the 

 island of Reil, the anterior commissure, and the superficial layer of the 

 optic thalamus (Valentin). 



The middle or grey root is of a pyramidal shape, and consists of grey 

 matter on the surface, which is prolonged from the adjacent part of the 

 anterior lobe and perforated space. Within it there are white fibres, which 

 have been traced to the corpus striatum. 



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, where they are said by Foville to be 

 connected with the longitudinal fibres of the gyrus fornicatus. 



The question whether the olfactory bulbs ought to be considered as nerves or as 

 cerebral lobes is, if tested by reference to the history of development, not so simple 

 as might at first appear. It is in favour of their being regarded as lobes, that in 

 the lower vertebrate animals the olfactory bulbs are generally recognised by com- 

 parative anatomists as additional encephalic lobes, and that in most mammals they 

 are much larger proportionally than in man, and frequently contain a cavity or 

 ventricle in their interior, and further that in their minute structure they nearly 

 agree with the cerebrum ; but, as it is known that in the first development of the 

 eye the peripheral part or retina, as well as the rest of the optic nerve, is originally 

 formed by the extension of a hollow vesicle from the first foetal encephalic compart- 

 ment, so in the case of the olfactory nerve, although the peripheral or distributed part 

 is of separate origin from the olfactory bulb, the latter part is comparable in its 

 origin with the optic vesicle. 



2. The second pair or optic nerves of the two sides meet each other at 

 the optic commissure (chiasma), where they partially decussate. From this 

 point they may be traced backwards round the crura cerebri, under the 

 name of the optic tracts. 



Each optic tract aiises from the optic thalamus, the corpora quadri- 

 gemina, and the corpora geniculata. As it leaves the under part of the 

 thalamus, it makes a sudden bend forwards and then runs obliquely across 

 the under surface of the cerebral peduncle, in the form of a flattened band, 

 which is attached by its anterior surface to the peduncle ; after this, becoming 

 more nearly cylindrical, it adheres to the tuber cinereum, from which and, as 

 stated by Yicq-d'Azyr, from the lamina cinerea it is said to receive an 

 accession of fibres, and thus reaches the optic commissure. 



In the commissure the nerve-fibres of the two sides undergo a partial de- 

 cussation. The outer fibres of each tract continue onwards to the eye of the 



