408 SAMUEL C. PALMER 
in the line joining two cones,” and ‘‘ Toward the extreme periph- 
ery of the retina the cones become more numerous and close 
to the ora are alone present.”’ Hesse (’00) gives the total number 
of visual cells in the human retina as 50,000,000. Stohr (06) 
believed that the rods were more numerous than the cones and 
that the latter occurred at regular intervals, so that three or four 
rods lay between adjacent cones. Pitter (02) has given a great 
deal of attention to enumerations of the histological elements 
in the retina and in the optic nerve of marine mammals. The 
total number of elements in the different layers of the retina 
was found to be enormous (table 1). Estimates of the number 
of rods ranged from 227,000,000 in the adult Phoca vitulina to 
800,000,000 in Macrorhinus leoninus and Balaenoptera physalus. 
As no mention was made of cones, except in the case of Hyper- 
' oodon rostratus, where they were said to be wanting, the retinas 
were presumably pure rod retinas. The nuclei in the outer nu- 
clear layer were said to be from six to thirteen times as numerous 
TABLE 1 
Estimated numbers of retinal elements and optic nerve fibers in certain species of 
marine mammals (after Pitter, ’02) 
| 
| 
2 1 c NUMBER OF NUCLEI PER | NUMBER OF OPTIC | , 2 
z al & a = SQUARE MILLIMETER IN NERVE FIBERS g a 
SPECIES 6 5 a S S - ok wp 
eee & < Pp = QA 
ers = s is Outer Inner orl . oa < 
gag | yaw | nuclear nuclear earn Total HHO 
5Sae <A & | layer layer mil <7 & 
z si 5 e meter | Pe 
Macrorhinus | | 
leoninus.......| 90,000 , 800,000,000 1,250,000} 110,000 | 103 767,000 |1:1050 
Phoca barbata ..| 120,000 | 363,000,000 1,367,000, 119,000 68 | 174,000 |1:2086 
Phoca vitulina.. | 110,000 | 227,000,000 | 1,512,500/ 78,000] 74 147,000 /1:1544 
Odobaenus | 
6,000,000 | 722,000) 82,000 62 | 111,000 
rosmarus......| 110,000") 25 1:23800 
Otaria jubata....| 250,000 | 475,000,000 | 2,000,000 181,000 74 | 140,000 |1:2000 
Balaenoptera | 
physalus...... - 62,000 | 800,000,000} 550,000, 62,000 | 13 | 157,000 /1:5095 
Phocaena com- | | | 
munis........ - 200,000 | 175,000,000 | 1,350,000, 184,000 | 29} 36,100 /1:4850 
Delphinapterus | | 
leucas.... | 150,000 | 735,000,000 794,000 98,000 | 28 | 137,000 |1:5560 
Hyperoodon 
rostratus.. 111,000 | 557,000,000 918,000, 90,000 | 
— 
~ 
77,000 1:7200 
