366 CLASS XVI. 



sphere, and hence the curvature of the cornea is much greater than 

 that of the sclerotic coat in the base of the eye. When the poste- 

 rior segment is large the two segments are united by a narrow 

 margin running obliquely forward; if, on the contrary, the posterior 

 segment be only a small portion of a sphere, then this uniting 

 margin is elongated to form a truncated cone, and the axis of the 

 eye becomes much longer, as, for instance, in the owls. At this 

 part the sclerotic coat receives between its laminte a ring of bony 

 plates or scales, of which the number differs in different birds, but 

 is commonly fourteen or fifteen^ Fibres of striped muscle proceed 

 forward from these scales and are inserted into the inner lamina of 

 the cornea^. Their action will increase the convexity of the cornea. 

 The vascular membrane [clioroidea) is covered abundantly with a 

 black pigment. As an arrangement in the eye peculiar to birds the 

 black fan or comh {jjecten) must be noticed, which consists of a fold 

 of the vascular membrane ichoroidea) , and may be regarded as a 

 higher development of the sickle-shaped band in the fish's eye (see 

 above, p. 48), and of the fold present in many lacertine animals 

 (see above, p. 232). This fan proceeds from the entrance of the 

 optic nerve into the eye-ball obliquely upwards to the axis of the 

 eye, and in many extends as far as the capsule of the lens. The 

 number of the folds is various; they are most numerous in the 

 Passeres, from twenty to thirty; on the other hand they are fewest 

 in number in the nocturnal birds of prey and Caprimulgus (five to 

 seven) ^. This part does not contain muscular tissue, and hence 

 the only effect it can produce by changing its circumference must 

 be caused by the greater or less distension of the blood-vessels 

 which are very numerous in it. On the probable use of the fan 

 many surmises have been formed; we here notice only the opinion 



1 A full description of this bony ring has been given by J. A. Albers Beitrdge 

 zur Anatomie «. Physiologie der Thiere, istes Heft, Bremen, 1802, 4to, s. 73 — io6, 

 •with figures. In many birds there is also another ossification in the sclerotica, a bony 

 plate in form of a ring or horse-shoe, at the entrance of the optic nerve into the bulb. 

 See Gemmingek Zc/tec/<?'. /. ivissensch. Zool. iv. 1853, p. 215; F. Leydig, Mueller's 

 Archiv, 1855, pp. 40 — 46, Taf. vi. figs, i — 7. 



2 These fibres form the musculus cramptonianus, so named after Crampton, its 

 discoverer. Tbomfsoh's Annals of PhilosophT/, 18 13, p. 172. 



^ Compare Wagner Beitr, zur Anal, der Vogel. (1. 1.) s. 295 — 300. In Apteryx 

 this part is wanting, according to Owen Zool. Transact. 11. p. 293 ; this is a single 

 exception. See also on the pecfen JEa. Huschke Commentatio de Pectinis in ocuJo 

 Avium potestate anatomica et physioloyica. Accedit Tab. sen. Jense, 1827, 4to. 



