610 



PHYSIOLOGY 



at various distances on to the retina, practically every possible focusing method 

 is made use of in one type or another of the animal kingdom. The following 

 details are taken from Beer's papers. 



In birds the eye, like that of man, is normally focused for distant objects, 

 and accommodation for near objects is accomplished by a change in curvature 

 of the anterior surface of the lens. Whereas, however, in man the suspensory 

 ligament is relaxed by a drawing forwards of the choroid membrane, in the 

 bird's eye this relaxation is effected by a drawing backwards of the posterior 



A 



J--- 



FIG. 274. Diagrams from Beer to show mode of accommodation 



distance) in a fish. 

 A, vertical section of the eyeball ; B, view of eye from front ; L, lens ; 

 Ls, suspensory ligament ; 7, iris ; Rl, retractor lentis or ' campanula ' ; 

 C, changes in position of lens when eye is accommodated for an object at 

 varying distances. 



lamina of the cornea, where it breaks up into the ligamentum pectinatum iridis. 

 In these eyes the main attachment of the suspensory ligament is to the liga- 

 mentum pectinatum ; the retraction of this ligament is effected by a special 

 muscle known as Crampton's muscle, which corresponds to the ciliary muscle 

 in man, but unlike this consists of striated voluntary muscle. This movement 

 of the posterior elastic lamina of the cornea can be easily shown by passing 

 two needles through the eorneo-sclerotic junction until their points lie in the 

 anterior chamber. On exciting Crampton's muscle electrically, the outer 

 end of the needle moves forwards, showing that the deeper part of the eorneo- 

 sclerotic junction is being pulled backwards towards the ciliary portion of the eye 

 (Fig. 273). The histological character of the muscle of accommodation in birds 



