TEE APPARATUS OF SMELL. 92$ 



with the cavities it lines, the olfactory apparatus. These parts having been already 

 referred to at p. 523, we will pass to the other senses. 



(The olfactory filaments, passing down from the olfactory ganglion, form a 

 plexus upon the surface of the pituitary membrane. These filaments, as already 

 noted (p. 523), differ widely from those of the ordinary cranial nerves, in containing 

 no white substance of Schwann, but are nucleated and finely granular in structure, 

 and resemble greatly the gelatinous form of nerve-fibres. Their distribution is 

 limited to the membrane at the upper third of the nasal septum, the upper part 

 of the turbinated bones, and the wall of the nasal cavities adjoining the cribriform 

 ^late of the ethmoid bone ; all this surface being covered with an epithelium of 

 a rich sepia-brown hue. As has also been mentioned, Schultze divides these cells 

 into two sets — supporting cells and olfactory cells. The first (Fig. 309, a) are 

 described as terminating externally by truncated flat surfaces, which cannot be 

 observed to be covered by any membrane separate from the contents of the cell. 

 These contents appear to consist of protoplasma with a yellow granular aspect 

 externally, wliile at the lower part an oval nucleus embedded in transparent 

 protoplasm can be easily seen. At their attached end, these cells become 

 attenuated, and may be traced inwards for a considerable distance until they 

 expand into a broad flat sheet or plate, which is never coloured, though it 

 frequently presents a granular appearance. The processes passing off from this 

 appear to be continuous with the fibres of the submucous connective tissue. 

 Towards the margin of the true olfactory region, cells perfectly analogous to 

 these are met with, the only difference being that they present a well-defined 

 band or seam at their free extremity, which is surrounded by a circle of ciha 

 {Fig. 309, c). The cells of the second set (Fig. 309, b) are continuations of the 

 nerves, and have been named olfactory cells. They are thin, fibrous, or rod-like 

 bodies, terminating at the same level as the proper epithelial cells, and presenting, 

 when traced inwards, a series of varicose swellings directly continuous with the 

 prolongations of deeper-seated nerve-cells. Clarke states that the nerve-fibres, 

 on reaching the base of the epithelial layer, divide into finer and finer branches, 

 to form a network with numerous interspersed nuclei, through which they are 

 probably connected with the olfactory cells (Fig. 309,/). The proper epithelial 

 cylinders {d, e) are connected at their bases with the septa formed of the connec- 

 tive tissue belonging to the subepithelial glandular layer.) 



CHAPTER IV. 



APPARATUS OP VISION. 



Designed for the perception of external images rendered visible by the luminous 

 rays, the sense of sight depends upon the excitability of the optic nerve, the 

 terminal extremity of which is expanded as a thin membrane at the back of 

 each eye. The latter is a globular organ lodged in the orbital cavity, attached 

 to muscles which can move it in various directions, and protected by membranous 

 and movable screens known as the eyelids, the play of which over the surface of 

 the eye is facilitated by the lachrymal fluid ; this keeps their inner surface 

 constantly moist. 

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