756 



SPECIAL SENSES. 



and it arises from the most posterior portion of the anterior lobe. The middle, or gray 

 root arises from a little eminence of gray matter situated on the posterior and inner por- 

 tion of the inferior surface of the anterior lobe. 



The deep origin of these three roots of the olfactory nerves is still a matter of discus- 

 sion. The external root is stated by various anatomists to originate from the corpus 

 striatum, the optic thalamus, the anterior commissure, and the island of Reil; but 

 researches upon this point have been by no means satisfactory. The same uncertainty 

 exists with regard to the deep origin of the internal white root and the gray root. 



The three roots of the olfactory converge to form a single nervous cord at the inner 

 boundary of the fissure of Sylvius. This passes forward and slightly inward in a deep 

 groove between two convolutions on the under surface of the anterior lobe, covered by 

 the arachnoid membrane, to the ethmoid bone. This portion of the nerve is exceedingly 

 soft and friable. It is composed of both white and gray matter, the proportions being 

 about two-thirds of 'the former to one-third of the latter. The gray substance, derived 

 from the gray root, is situated at the upper portion of the nerve, the white substance 

 occupying the inferior and the lateral portions. 



By the side of the crista galli of the ethmoid bone, the nerve-trunk expands into an 

 oblong ganglion, called the olfactory bulb. This is grayish in color, excessively soft, and 

 contains the ordinary ganglionic elements. From the olfactory bulb, from fifteen to 

 eighteen nervous filaments are given off, which pass through the foramina in the cribriform 

 plate of the ethmoid bone. These filaments are composed entirely of nerve-fibres and are 

 quite resisting, owing to fibrous elements prolonged from the dura mater. It is strictly 



proper, perhaps, to regard these as the true olfactory 

 nerves, the cord leading from the olfactory bulb to the 

 cerebrum being more properly a commissure. Having 

 passed through the cribriform plate, the olfactory nerves 

 are distributed to the olfactory membrane in three groups : 

 an inner group, distributed to the mucous membrane of 

 the upper third of the septum ; a middle group, to the 

 upper portion of the nasal fosses ; and an outer group, to 

 the mucous membrane covering the superior and middle 

 turbinated bones and a portion of the ethmoid. 



The mode of termination of the olfactory nerves differs 

 from that of the ordinary sensory nerves, and is peculiar 

 and characteristic, as it is in the other organs of special 

 sense. According to recent observations, the olfactory 

 mucous membrane contains peculiar terminal nerve-cells, 

 FIG. ^.-Terminal filaments of the called the olfactory cells, which are situated between the 

 olfactory nerves; magnified 30 cells of epithelium. These are long, delicate, spindle- 



diameters. (Kolliker.) 



i, from the frog.-o, epithelial cells of shaped structures, varicose, each one containing a clear, 

 the olfactory region; b, olfactory round nucleus. The appearance of these, which are con- 

 ceils. 2. small branch of the olfac- . , 



tory nerve of the ftopr, separating a t sidered as the true olfactory organs, is represented m Fig. 



fibril"! S&toSu of th v e a eheep! 234. In the frog, there is a fine, hair-like process projecting 

 from each cell beyond the mucous membrane, which has 



not been obsetved in man or the mammalia. The great delicacy of the structures enter- 

 ing into the composition of the olfactory membrane renders the investigation of the ter- 

 mination of its nervous filaments exceedingly difficult. 



Properties and Functions of the Olfactory Nerves. It is almost certain that the olfac- 

 tory nerves possess none of the general properties of the ordinary nerves belonging to the 

 cerebro-spinal system, but that they are endowed with the special sense of smell alone. 

 As far as we know, no one has exposed and operated upon the filaments coming from the 

 olfactory bulbs and distributed to the pituitary membrane in living animals ; but experi- 



