DE. COBBOLD ON THE EYE OF THE COD-FISH. 



147 



author last named considered them equivalent to the pseudo- 

 naviculae of Gregarina, whilst Dufour provisionally placed them 

 among the Entozoa, with which, however, they have evidently no 

 legitimate connexion. Caustic potass dispels their colour, and, 

 without entirely bursting, they display peculiar jerking movements, 

 apparently due to the escape of sarcode at one or more points ; 

 but they do not, nevertheless, undergo any change of form. 



(2.) According to the early investigations of Haller, Hunter, 

 Cuvier, and others, the so-called choroid gland is prevalent in osse- 

 ous fishes generally ; and I believe this statement is correct ; but 

 their individual opinions as to its nature and function are very 

 various. Hunter thought it muscular; Sommering doubted 

 whether it were glandular, vascular, or muscular ; and Cuvier took 

 up with the notion that it was to be classed with erectile tissues. 

 But what are the facts which microscopical inquiry discloses ? If 

 a thick vertical or horizontal section be removed from this gland 

 (which is horseshoe-shaped in the Cod) and placed under the 

 quarter-inch objective, the arterial and venous trunks going to it 

 will be found to divide suddenly into multitudes of minute capil- 

 laries, the latter therefore taking their origin from the line of de- 

 marcation which indicates the inner border of the so-called gland. 

 The small capillaries will be further found to be intimately blended 

 together by their own walls, and not by the intervention or exten- 

 sion of any fibres from the connective tissues of the choroid mem- 

 branes. They are all arranged in a simple linear, parallel manner ; 

 and their width does not appear to exceed that of the short diameter 

 of the blood-corpuscles, the admeasurements of the latter being 

 about 2 5 ] th of an inch long, by o o*h °f an ^ breadth. 

 In fresh eyes the capillaries are always found gorged with blood ; 

 and when I recently succeeded in isolating, more or less com- 

 pletely, a few of the vessels of the band, one of them was seen to 

 contain blood-corpuscles arranged in single file. The capillaries 

 are straight and of uniform diameter throughout, and they do not 

 give off any branches or dilatations such as are found to occur in 

 the true erectile organs. 



(3.) Some seven or eight distinct nervous layers have been in- 

 dicated as together constituting the retina ; but for all practical 

 purposes I think it sufficient to recognize four, namely, Jacob's 

 membrane, the soft internal layer (consisting of various laminae of 

 different-sized cells and granules held together by the so-called 

 Mullerian filaments), the fibrous expansion of the optic with its 

 vessels, and, lastly, the thin hyaloidal cellular layer. Confining our 



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