816 



TSE APPABATVS OF THE SENSES. 



Fig. 381. 



one (Fig. 382, a) being described as terminating externally by truncated 

 flat surfaces, wbich 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, while at the lower 

 part an oval nucleus imbedded in transparent protoplasm 

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

 come attenuated, and may be traced inwards for a con- 

 siderable distance until they expand into a broad flat 

 sheet or plate, which is never coloured, though it fre- 

 quently 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 

 FIBRES OP ULTIMATE cilia (Fig. 382, c). The cells of the second set (Fig. 

 EAjiiMOATioNs or 382^ ])) are continuations of the nerves, and have been 

 o^^^^TOEY NERVES ^^^^^ olfadoTy 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. 

 382,/). The proper epithelial cylinders (d, e) are 

 connected at their bases with the septa formed of the 

 connective tissue belonging to the subepithelial glan- 

 dular layer.) 



Fig. 382. 



CHAPTEE IV. 



APPAEATUS OF VISION. 



Designed for the perception of external images ren- 

 dered 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 dn various directions, 

 and protected by membranous and movable screens 

 CELLS OP THE OLFAOTOEY knowu as the eyelids, whose play over the surface of 

 MUCOUS MEMBEANE. the eye is facilitated by the lachrymal fluid, which 



a, 5, c, After Schultze ; j^. -^ ^j^^^^ ^^^gj, g^j,fg^^,g ^^^^^.^^^^2 jjjg|._ 



0, e, f, After Lockhart ■' , .. , jr ■ ■ i i j^ ^i 



Clarke. ^■'^^ essential organ oj vision, or globe oj the eye, 



■will be first described ; then, under the designation 



of the accessory portions of the visual ai:iparatus, we vcill notice the receptacle 



