BIRDS 



405 



and most fully studied by Kajikawa (1923), are most marked near the 

 fovea. It may be that they regulate the amount of blood in the 

 choroid which in Birds is particularly distensible, swelling remarkably, 

 for example, and becoming intensely engorged if the intra-ocular 

 pressure is suddenly lowered by paracentesis of the anterior chamber 

 (Abelsdorff and Wessely, 1909) ; others, again, consider that their 

 contraction adjusts the position of the fovea in accommodation, acting 

 after the manner of a fine adjustment of a microscope. 



In the Picidffi (woodjoecker, Colaptes) the sinusoidal choroidal layer is 

 filled with mucoid tissue, as if to provide a cushion against the repeated mechanical 

 trauma of wood-pecking (Walls, 1942). Birds have no tapetum ; the " eye- 

 shine " seen in some species has been attributed to a reflex from Bruch's 

 membrane (ostrich, Struthio). 



The vascular layer of the choroid is continued forwards into the 

 ciliary region without the intervention of an orbiculus, the whole zone 

 being occupied by the numerous elongated ciliary processes ; ventrally, 

 in the region of the fa?tal cleft, it is claimed that a particularly marked 

 CILIARY CLEFT between the processes allows communication between 

 the anterior and posterior chambers (Niissbaum. 1901 ; Hess, 1912 ; 

 Ischreyt, 1914). The ciHary processes and their associated uveal tissue 

 angle sharply inwards to approach the lens, while the ciHary muscles 

 cling closely to the sclera, thus separating the two components of the 

 ciliary body and leaving a deep cleft-like space bet^^ een the two layers 

 traversed by the strands of the pectinate ligament (Fig. 499). The 

 ciliary musculature, which is made up of striated fibres, reseml)les that 

 of the lizard in its topography ^ (Fig. 500) ; both it and the muscles of 

 the iris are supplied by a complicated plexus of motor and sensory 

 nerves (Boeke, 1933). The meridional muscular bundle apj^ears to be 

 divided into two ; anteriorly the muscle of cramptox. a stout 

 muscular band, arises from the inner sm^face of the cornea at its 

 margin and is inserted into the sclera as it bitlges axially in the ciliary 

 region ; more posteriorly brucke's muscle, arising from the inner 

 aspect of the sheet of sclera which forms the anchorage of the pectinate 

 ligament, is inserted into the posterior portion of the ciliary body, an 

 insertion A\hich is prolonged to the sclera by the texacular ligament, 

 thus relieving the choroid of mechanical strain. Accommodation, as 

 in lizards, is mainly effected by the contraction of the meridional 

 musculature forcing the ciliary body against the lens so as to deform it, 

 tautening the fibres of the pectinate ligament meanwhile (Wychgram, 

 1913-14). Simultaneously the stout Crampton's muscle running from 

 the cornea to the sclera like a bow^-string, deforms the cornea and 

 shortens its radius of curvature, an action much more pronounced in 

 Birds than in lizards. 



1 p. 3.57. 



The ostrich 

 Struthio 



