ORGANS OF THE BODY. 



635 



layer perpendicularly, or in other words, in a direction convergent 

 towards the centre of the eyeball Eventually it terminates in a body 

 known as its "granule" (&'). We shall be obliged to refer to it again. 



The rods of the retina, like the cones, belong to those few tissue 

 elements of the animal body, which manifest characteristic differences 

 among the various classes of living creatures. In the eyes of naked 

 amphibians, such as frogs, toads, and salamanders, they are enormously 

 large. 



The great tendency to change after death, manifested by the rods, 

 renders it a difficult point to decide how far many other structural pecu- 

 liarities recently described by numerous observers really exist in the living 

 retina. 



The first point to be borne in mind is, that the internal member of the 

 column is not found to be homo- 

 geneous among all the mammalia. In 

 connection with the larger rods of the 

 batrachia (fig. 590, 2, 3), of fishes 

 (4), and even of birds (1), we meet 

 namely, with remarkable lenticular 

 bodies of hemispherical or planopara- 

 bolic figure, whose flat sides are 

 applied to the bases of the external 

 members (a, a). These structures 

 are extremely rapid in decomposition 

 (Schultze). They have been named 

 " bacillary elipsoids" (Stabchen elip- 

 soide) by Krause. 



It has been long known (but the 

 process has been recently more fully 

 investigated by Schultze) that the ex- 

 ternal halves of the rods may resolve 

 themselves (fig. 591, 5) into a number 

 of transverse plates, or if the process of 

 decomposition have progressed farther, 

 into thin disks, resembling slightly 

 those of the muscle fibres. The transverse markings are, in man and the 

 mammals, about 0*0003-0 "0004 mm. distant from one another (Schultze). 



The external members of the rods present further, as has been known 

 for years (Hensen, Schultze), a peculiar longitudinal striation. Transverse 

 sections in the frog show that this is produced by a longitudinal grooving, 

 running to the extent of internal cleavage. 



However and here we tread upon very uncertain ground the in- 

 ternal joints of the rods in man, and many mammals (fig. 591,1,3), present 

 superficial longitudinal striation too. This is, perhaps, directly continu- 

 ous with the longitudinal cleavage rifts of the external members. It 

 appears to correspond to a delicately grooved connective-tissue investing 

 lamina, which Would therefore represent an outward prolongation of the 

 membrana limitans externa. Quite recently this has been named by 

 Schultze. the "fibre-basket." According to this observer, however, the 

 internal joint of the rod presents, likewise, a fibrillated structure, and, 

 moreover, in the interior. 



We shall now enter somewhat further into the consideration of the 

 various views entertained upon this very obscure subject. 



Fig. 590. Structure of the rods of the retina. 

 1, From the hen; 2, from the frog; 3, from 

 the salamander ; 4, from the pike, showing 

 external and internal members, and in the 

 latter the lenticular bodies referred to in the 

 text ; 5, resolution of the external portion of 

 a column into disks, from the frog (after 

 Schultze). 



