SECT. 226.] 



JACOB S MEMBRANE. 



555 



a time: (acetic acid, added to the rods of the frog's retina, causes 

 them to swell up to two or three times their original size, and 

 generally makes them roll up.) Concentrated acetic acid, as well 

 as alkalies and mineral acids, dissolve them in a short time, while, 

 on the other hand, diluted chromic acid is their best preservative, 

 although it causes them to shrivel somewhat; Fig. 226. 



with strong syrup and sulphuric acid they be- ' 



come red, while nitric acid and potash colour 

 make them yellowish. — Taking all together, we 

 may, perhaps, he allowed to. regard the main 

 constituent of those structures as a protein sub- 

 stance, and to look upon the rods themselves 

 as delicate tubes, with viscid albuminoid con- 

 tents. 



The cones (fig. 225, 1) are somewhat shorter 

 rods, provided at their inner extremity with 

 a conical or pyriform body, whose length 

 (0-007'" t° o , oi5'") is equal to half the thick- 

 ness of the bacillar layer, and measures o , oo2o'" 

 to o , O03o'" in breadth. Each of these cones 

 consists, when fresh, of an almost homogene- 

 ous or extremely finely granular substance, with 

 a slight lustre resembling that which forms the 

 rods, except that it is clearer ; it undergoes 

 changes with almost the same facility, especially 

 having a great tendency to swell up. The rods, 

 which are continued externally from the cones, 

 or the cone-rods, are sometimes just as long as 

 the free rods, sometimes a little shorter. Un- 

 like the rods, however, the cones are continued by a constricted 

 portion into a pyriform swelling, 0'003'" long, containing a cell- 

 nucleus ; this may be termed the cone-granule, and it lies in the 

 outer granular layer, being united by a fine filament, similar to 

 that of the rods, with the inner parts of the retina. 



The rods and the cones are situated close to one another, and 

 are arranged perpendicularly upon the retina, in the manner of 

 palisades, the one extremity being directed outwards towards the 

 choroid, the other towards the granular layer. The cones form an 

 almost continuous layer (fig. 227, 2) in the neighbourhood of the 

 yellow spot, the rods being arranged only in simple series between 

 them ; further forwards, however, they separate from each other, 

 being distant at first, 0'002'" to o , oo3'", and in the anterior parts 



Altered elements of the 

 bacillar layer of the hu- 

 man retina. I. rods de- 

 tached from their fila- 

 ments; variously notched, 

 curved, and varicose, 

 some of them broken; 2. 

 two cones enlarged by 

 chromic acid, showing 

 their granular contents 

 and shining nucleus, one 

 of them with a shortened 

 rod, the other with a rod 

 swollen at the extremity; 

 a. rods; 6. cones; c. nu- 

 cleus ; d. fibres of Milller 

 torn off. Magnified 350 

 times. 



