ENCEPHALIC NERVES. 



453 



Fig. 299. 





are much altered by all reagents; and a very common change con- 

 sists in their end presenting a hook-like curve or a slight enlarge- 

 ment. (Figs. 298 and 295, ra'.) 



The cones may be regarded as rods terminating internally in a 

 conical or pyriform body (instead of a filament), whose length 

 equals one-half the thickness of the bacillar layer, and its breadth 

 being -Q\-Q to ^^5$ of an inch. Each of these cones has an exter- 

 nal thicker and larger, finely granular extremity, often ventricose, 

 gradually diminishing in size, and passing into a common rod with- 

 out a point ; and a shorter inner portion, inclosing an elongated or 

 pyriform, more opaque, and brilliant body, 

 euV u to 4 oVo f an mcn l n g- Kolliker, how- 

 ever, sees in these cones, as just described, 

 only a cell with a nucleus. (Fig. 298, 4.) 



The rods and cones are arranged vertically 

 upon the retina, like palisades in close appo- 

 sition, one of their ends being directed to- 

 wards the choroid membrane, and the other 

 towards the granular layer. The appearance 

 of the cones and rods when the bacillar layer 

 is seen from without (the cones alone existing 

 over the macula lutea), is shown by Fig. 299. 



3. The eighth pair is the nerve of hearing. 

 The acoustic nerve is composed of fibres g^^ 

 to JZ^TJ-Q of an inch in diameter, which are very 

 easily destroyed, and are invested by a very 

 delicate perineurium. Hence it has been 

 called the portio mollis. Among these fibres in the trunk, and in 

 both the vestibular and cochlear branches, there occur numerous 

 apolar, unipolar, and bipolar cells, gj^ to T 4 T of an inch in diame- 

 ter. Kolliker suggests that the first two kinds are truncated bipolar 

 cells. Similar but smaller cells are also found in the cochlea, as 

 well as in the nervous twigs in the vestibule. 



The vestibular nerves finally break up into a rich bundle of smaller 

 and frequently anastomosing branches, which appear to terminate 

 ultimately in fine twigs composed of from two to ten primitive 

 fibres, 72^(7^ ^ s^W f an inch thick. In the sacculus we find the 

 otoliths in immediate relation with the nervous expansion. These 

 are composed of innumerable hexahedral prisms of carbonate of 

 lime. & 



Bacillar layer from without. 

 1. At the "yellow spot" (only 

 cones). 2. At the border of 

 the same. 3. From the mid- 

 dle of the retina, a. "Cones," 

 or vacuities corresponding 

 with them. b. "Kods" of the 

 " cones," whose terminal sur- 

 face is often placed rather 

 more deeply than that of the 

 proper "rods," c. Magnified 

 350 diameters. (Kolliker.) 



