ORGANS OF THE BODY. 



615 



3 



is formed. Between those cylindrical elements there remain naturally 

 interstices, which serve for the reception of another species of cell, to be 

 referred to again immediately. The occurrence of particles of yellowish 

 or brown pigment, either in the upper broader portion of the columnar 

 cells (fig. 573, 2 a), or deeper down below their nuclei (fig. 572, c), is 

 peculiar ; these granules may not unfrequently be seen also in the widened 

 portions of the processes (fig. 539, c). The former is the case in man 

 and many of the mammalia. Like the contents of the glands of Bowman, 

 these coloured particles communicate to the locality in which they are 

 situated a peculiar tint. 



Between these decidedly epithelial elements (and moreover, in all verte- 

 brate animals) a second species of cell presents itself (fig. 573), different 

 in shape and composition, and of nervous nature. These cells consist of a 

 fusiform body (fig. 573, 1 b, 2 b), situated deeper down, however, than the 

 bodies of the first kind, containing a vesicular nucleus and finely granular 

 contents. From both poles of each of these nervous structures, as they are 

 held to be, these olfactory cells, a process is 

 given in opposite directions. The descend- 

 ing (fig. 573, Id, 2 d) is of extreme deli- 

 cacy, and liable to undergo early decom- 

 position. At intervals it is studded with 

 minute varicosities, recalling to mind those 

 of very fine nerve tubes ( 176). The 

 ascending process (1 c, 2 c) is, on the 

 other hand, thicker and less knotted, 

 presenting a more or less smooth outline. 

 It has the form of a slender cylinder or 

 rod, 0-0018 or 0-0009 mm. in diameter, 

 reminding us of one of the elements of 

 the retina, to be referred to presently. 



These rods mount up between ' the 

 columnar epithelial cells to the surface 

 of the mucous membrane, terminating 

 here in various ways. In the frog and 

 allied amphibians (in which they may 

 be easily seen), they are surmounted at 

 their free extremity (fig. 573, 1 e) by a set 

 of delicate hairs of considerable length, in 

 a certain number of which ciliary motion 

 has been observed, while others, gene- 

 rally the longest, remain quite stiff. These 

 two kinds of " olfactoi-y filaments" appear 

 to be linked together by intermediate 

 forms. In other amphibia and birds 

 very similar, and in certain cases, even 

 longer hairs are to be found, either in 

 large numbers or single (Schultze) none 

 have been observed among the fishes, 

 mammalia also, we may seek in vain for these paradoxical cilia. Here 

 we only remark small appendages, about from 0*0023 to 0-0045 mm. in 

 length, seated on the extremities of each rod (fig. 573, 2e), and projecting 

 beyond the ends of the columnar cells ; these, however, are only artificial 

 productions. 



40 



Fig. 573. 1. Cells from the rcgio olfactoria 

 of the frog. a. an epithelium eel! 

 dwindling below into a ramifying pro- 

 cess; ft, olfactory cells, with descending 

 fbres rf, peripheral rods c, and long 

 cilia, e. 2. Cells from the same locality 

 in man. The letters and description are 

 the same here, only tliat little (artificial) 

 appendages appear at e, upon the tips of 

 the rods. 3. Nerve-fibres of the olfac- 

 torius from the dog, breaking up at a 

 into fine fibrillse. 



In man, and the rest of the 



