1869.] 401 [Jeffries. 



ralist," in reference to the method of accommodation in the 



eyes of birds. 



The refractive power of the eye must be increased to focus on the 

 retina the diverging rays of Hght coming from near objects. This 

 is there stated to be done by the sweUing up of the marsupium or 

 peeten, and its pressure on the lens behind, the external muscles 

 compressing the globe and rendering the cornea more convex. The 

 error of statement is perhaps due to what Prof Owen has said in his 

 Comparative Anatomy of Vertebrates, who does' not seem to be aware 

 of, at least has not noticed, the investigations of recent observers on 

 this point, either in man or the lower animals. He refers to Cramp- 

 ton's observations in 1813, upon the muscle in the interior of the eye 

 discovered by him, and since then bearing his name. 



Crampton's theory was that this muscle by its action flattened the 

 cornea. Prof. Briicke, in studying this muscle, came to exactly the 

 opposite conclusion. In man, accommodation takes place by the 

 crystalline lens becoming more convex on its anterior surface. This 

 is done through the action of the ciliary muscle, exactly how is not 

 yet proved. In the act of accommodation or bringing diverging rays 

 to a focus on the retina, the cornea does not change its curve, the iris 

 has nothing to do Avith it, the lens does not change its position but 

 simjily its shape, and this through the action of the ciliary muscle. 

 Unity of design would lead us to expect a similar method of action 

 in those animals which, from their habits, we judge need the power of 

 active and rapid accommodation. So far experiment and observation 

 seem to confirm this. The peeten or marsupium cannot be an active 

 agent in accommodation, since in many birds it is quite rudimentary, 

 not reaching the lens at all. Its tissue is vascular, not muscular. 

 The external muscles of the globe can not affect accommodation, 

 since it takes place when the eyeball is removed. The experiments 

 of Cramer and Trautvetter show that the cornea does not change its 

 curve during accommodation ; the latter observer found also that the 

 lens changed its shape as in man, that the iris did not affect it, and 

 finally that the motor oculi nerve was the nerve of accommodation. 

 This nerve supplies the ciliary or Crampton's muscle within the globe, 

 and thus we have the same elements in the bird's eye as in man. 

 Briicke, Donders, Cramer, Mannhardt and MUller, have specially 

 studied this muscle in its various development in animals. There has 

 been considerable dispute in reference to its anatomical relation In 



PROCEEDINGS B. S. N. H.— VOL. XII. 26 M.\Y, 18'J9. 



