636 



MANUAL OHF ISTOLOGY. 



In the year 1860 a very fine fibre was said by Hitter to exist, running, 

 through the axis of the rod. This, lie alleged, 

 terminates externally with a slight swelling, 

 leaving the hollow rod internally in the form of 

 the bacillary thread, already referred to. This 

 statement has received confirmation from Manz, 

 Spiess, Hensen, and Hasse, but its correctness 

 has been questioned by Krause, Hulke, Steinlin, 

 and others. In fig. 592, 2, we have given a 

 representation of these "fibres of Ritter" after 

 /Schultze. Their existence in the recent retina 

 may be regarded as still undecided. 



Turning now to the cones (fig. 588, B, c), we 

 find a still more remarkable structure than in the 

 last. In man these bodies present the shape of 

 slender flasks, whose bases are seated upon the 

 external limiting membrane. At their narrower 

 nds they run into somewhat pointed rod-like 

 structures, of the most extreme delicacy, and very 

 prone to rapid decomposition. These are known 

 as the cone-styles (Zapfenstabchen). They cor- 

 respond to the outer halves of the so-nearly related 

 rods, and manifest a tendency to break up into 

 the same series of disks as the latter (fig. 591, 2 a ; 

 594, b). The inferior expanded portion (answer- 

 ing to the belly of the flask), the body of the cone, 

 is at one time stumpy and broad, at another, thin 

 and slender; from O0041 to 0'0067 mm. in 

 diameter. The cones of the macula lutea are 

 particularly slender; they will be referred to 

 again later on. Here also the same longitudinal 

 striation is presented to us (fig. 591, 2, b), which 

 we have seen in the analogous internal joint of the 

 rods. The interior is likewise observed to possess 

 a fibrillated structure. At their bases, immedi- 

 ately underneath the limitans (fig. 591, 2, d), is 

 situated the " cone granule" a slight constriction 

 intervening between the two. This body consists of a small oval or pyri- 

 form cell, possessing both nucleus and nucleolus, and belonging to the 

 external granular layer of the retina. The whole length of the cones is, 

 as a rule, somewhat less than that of the rods, and sometimes consider- 

 ably so, as in the retina of the pig (Schultze). 



In regard to the proportion of rods and cones in the human retina, 

 much difference has been remarked according to locality. In the macula 

 lutea, where the sense of sight is most acute, cones alone are to be found, 

 as was discovered by Henle (fig. 593, 1). Around this spot the cones are 

 still crowded, so that they are only encircled by single rows of rods 

 (2). Further forwards and externally they occur more rarely, and are 

 surrounded by several rings of rods (3). The number of the latter, 

 therefore, in the whole retina, exceeds by far that of the cones. 



The rods and cones of the ape tribe are similar in all points to those of 

 the human being. 



In most of our domestic animals, likewise, such as the ox, sheep, pig, 



Fig. 591. Fibrillar coating of 

 rods and cones. 1. Rods. 

 2. Cones (of man), a, Ex- 

 ternal ; 6, internal joint ; c, 

 bacillary threads; d, limit- 

 ans externa. 3. Rods (from 

 the sheep). The fibrillag 

 here overtop the internal 

 joint. The external mem- 

 ber is missing (after 

 Schultze). 



