EYE. 



[ 313 ] 



EYE. 



coimectod with the nerve-cells of the retina 

 by minute nerve-tubes. 



Their outer portions have been distin- 

 guished as the haciUi and cones; but the 

 vehole probably form one continuous system 

 of nerve-cells and tubes. 



The haciUi, regarded (fig. 211, 1 h k', 2) as 

 distinct bodies, are cylindrical, narrow and 

 elongated, of the same breadth through- 

 out, truncated externally, and terminating 

 internally in a more slender portion of the 

 fibre ; they are from 1-4-30 to 1-330" in 

 length and 1-15,000" in breadth ; near the 

 point of attachment to the fibre is a trans- 

 verse line. They are extremely delicate, 

 and easily broken or deformed. The cones 

 (fig. 211 i ^, 4 ^) are bacilU with a conical 

 or pyriform body, and are also very easily 

 inj ured. A slight constriction divides each 

 cone into two parts, the innermost of which 

 (tig. 211 Ji c) contains a nucleus. The cones 

 are from 1-GOOO to 1-4000" in diameter. 

 In most parts of the retina the cones are 

 surrouuded by several bacilli ; opposite the 

 macula lutea they alone form a continuous 

 layer ; whilst at its margins, single bacilli 

 intervene between the cones (fig. 212). 



Fig. 212. 



End view of the rows of bacilli and cones from the 

 ontside. 1, opposite the macula Intea, cones only ; 

 2, at its margins ; 3, at the middle of the retina, a, 

 cones or spaces corresponding to them ; b, bacilli of the 

 cones, the ends of which are often situated somewhat 

 beneath the level of those of the true bacilli, e. 



Magnified 350 diameters. 



Opposite the entrance of the optic nerve, 

 both bacilli and cones are absent. These 

 curious bodies are more distinctly seen in 

 many animals than in man (PI. 50. fig. 5), 

 and they present certain varieties: thus, 

 the cones are deficient in bats, the hedge- 

 hog, the mouse, the mole, and the sharks 

 and rays ; while the bacilli are deficient in 

 the reptiles. 



The inner ends of the radiating fibres 

 when overlapping each other, and especially 

 when swollen by the action of water, 

 present the appearance of a number of 



rounded or angular cells (fig. 210, c), for 

 which they were once mistaken. 



It is thus evident that, excepting the 

 layer of nerve-cells and that of the fibres of 

 the optic nerves, the retina cannot truly be 

 considered as composed of layers. The 

 series of bacilli and cones, when torn from 

 their connexion with the radial fibres, form 

 the so-called Jacob's me7nbrane. 



Between the nerve-fibres, from the outer 

 to the inner limiting membrane, lies the 

 connective supporting tissue ; this also 

 forms a considerable portion of the mole- 

 cular and granular layers. In modem 

 works, the fibres of this tissue are called 

 Miiller's fibres. But KoUiker has shown 

 that the true radiating or Miiller's fibres 

 are not dissolved by boiling, and are coloured 

 red by Schultze's test; so that they have 

 not the chemical composition of connective 

 tissue. 



We cannot enter into the physiology of 

 these radial fibres, which have been shown 

 to be the percipients of light. 



Crystalline lens. The crystalline lens is 

 contained in a capsule (fig. 207 q s), con- 

 sisting of a perfectly structureless, very 

 elastic membrane, the anterior half of 

 which is lined with a single layer of very 



Fig. 213. 



Fibres or tubes of the lens of the ox. 

 Magnified 350 diameters. 



