PISCES. 



1001 



distinctly shews the plicated retina continued 

 on to the surface of the ligament, which seems 

 to be covered with the nervous expansion.* 



Humours of the eye. The quantity of the 

 aqueous humour in a Fish's eye is comparatively 

 very small, owing to the flat shape of the cor- 

 nea and the almost perfect immobility of the 

 iris. The posterior chamber is, indeed, quite 

 deficient, the uvea of the iris being adherent to 

 the capsule of the vitreous humour ; and even 

 the anterior chamber is frequently materially 

 encroached upon by the protrusion of the 

 crystalline lens through the aperture of the 

 pupil. As a refracting medium it is evident 

 that the aqueous humour, being nearly of the 

 same density as the surrounding medium, could 

 have little effect in concentrating luminous 

 rays, this duty being principally assigned to the 

 powerful lens immediately behind it. 



The crystalline lens in Fishes is nearly of a 

 spherical form, thus presenting the converse as 

 regards its refractive power of what exists in 

 the eye of Birds. The size of the lens in these 

 aquatic animals is very great, so that it en- 

 croaches largely upon the chamber of the 

 vitreous humour, extending to more than half 

 way between the pupil and the back of the 

 cavity of the eyeball. Its consistence is very 

 great, and its nucleus so hard as to remain 

 transparent even after immersion in spirit of 

 wine. It is enclosed in a soft capsule, between 

 which and the surface of the crystalline lens is 

 a small quantity of fluid, and is fixed in a deep 

 depression in the fore part of the vitreous hu- 

 mour by a circular membranous zone derived 

 from the hyaloid tunic, which surrounds it like 

 the artificial horizon of a geographical globe. Sir 

 David Brewster, in an admirable paper on the 

 anatomical and optical structure of the crys- 

 talline lens,f gives the following interesting 

 particulars relative to its minute organization in 

 the class of Fishes. Its form is that of a prolate 

 spheroid, the axis of revolution being a little 

 longer than the equatorial diameter. This axis 

 is the axis of the eye or of vision. The body or 

 substance of the lens is enclosed in an exceed- 

 ingly thin and transparent membrane, called 

 its capsule ; and if this be punctured, a thickish 

 fluid flows from the opening ; but upon re- 

 moving the capsule altogether, this fluid is 

 found to constitute only the outer coat of the 

 lens, the substance of the lens growing denser 

 and harder as we approach the centre of it. 



The body of the lens is not connected with 

 the capsule by any nerves or filaments what- 

 ever ; on the contrary, it floats as it were within 

 the capsule, and on holding the lens in his 

 hand, Sir D. Brewster observed its axis of revo- 

 lution take a horizontal position whenever it 

 was placed in an inclined direction. This was 

 repeated several times with the same lens, 

 although the experiment was tried unsuccess- 

 fully with others. When the lens is taken out 

 of its capsule, and the softer parts removed by 

 rubbing it between the finger and thumb, a 



* Vide Preparation 1650, in the physiological 

 series of the Museum of the Royal College of Sur- 

 geons, London. 



t Phil. Transact, for 1833, p. 323. 



hard nucleus is obtained, which consists of 

 regular transparent laminae of uniform thick- 

 ness, and capable of being separated like those 

 of sulphate of lime or mica. 



When the surface of any lamina has been 

 examined before it has been detached, it has 

 the appearance of a grooved surface like mo- 

 ther-of-pearl ; and in large lenses it is often 

 easy to trace these apparent grooves or lines to 

 the two poles of the axis of revolution, the 

 fibres bounded by them being consequently 

 widest at the equator, and growing narrower 

 and narrower as they approach the poles. The 

 maximum breadth of these fibres is about the 

 5500dth part of an inch, but of course they 

 become gradually attenuated as they approach 

 the poles of the lens in either direction. 



Having thus determined the form and size of 

 the fibres which enter into the composition of 

 the crystalline lens, it remained to ascertain the 

 mode in which they were fastened together so 

 as to resist separation and form a continuous 

 spherical surface, and this was found to be 

 effected by a very curious mechanism, the con- 

 tiguous fibres being united by means of teeth 

 exactly like those of rack -work, the projecting 

 teeth of one fibre entering into the hollows be- 

 tween the teeth of the adjacent one. It was 

 further found that the fibres gradually diminish 

 in size towards the centre of the lens, and the 

 teeth in the same proportion, so that the num- 

 ber of fibres in any spherical coat or lamina 

 was the same from whatever part of the lens it 

 it was detached. In conclusion, Sir David 

 Brewster observes, " In the lens of a Cod I 

 found that there were 2000 fibres in an inch at 

 the equator of a spherical coat or lamina, whose 

 radius was 3 6 3 ths of an inch ; consequently there 

 must have been 2500 in the spherical surface. 

 If we now suppose that the breadth of each 

 fibre is five times its thickness, and that each 

 tooth is equal to the thickness of the fibre, or 

 that five teeth are equal in breadth to a fibre, 

 we shall obtain the following results for the 

 lens of a Cod four-tenths of an inch in dia- 

 meter : 



Number of fibres in each la- 

 mina or spherical coat . . 2,500 

 Number of teeth in each fibre 12,000 

 Number of teeth in each sphe- 

 rical coat 31,250,000 



Number of fibres in the lens . 5,000,000 



Number of teeth in the lens . 62,500,000,000 

 or, to express the result in words, the lens of a 

 small Cod contains five millions of fibres and 

 sixty-two thousand five hundred millions of 

 teeth. A transparent lens exhibiting such a 

 mechanism may well excite our astonishment 

 and admiration." 



The vitreous humour in Fishes is proportion- 

 ally less abundant than in other races of Verte- 

 brata, a circumstance which is partly owing 

 to the shortness of the antero-posterior dia- 

 meter of the chamber of the eye-ball, and partly 

 to the extent to which it is encroached upon by 

 the large spherical crystalline lens; in other 

 respects it presents no peculiarities worthy of 

 special description. 



Muscles of the eyeball. --The eyeball of 



