THE TELEOST EYE 583 



hyaloid vessels clinging to the inner surface of the retina (see also 

 p. 575). These are usually supplied from the same artery which would 

 otherwise go to the falciform process; but when the latter is absent the 

 artery enters the vitreal cavity at the disc, and branches over the retina, 

 instead of turning ventrally there to run through the chorioid along the 

 line of the embryonic fissure. Falciform process and hyaloid vessels are 

 thus mutually exclusive — a given teleost exhibits one or the other in 

 full bloom, never both. Since the vitreal vessels are clearly concerned 

 with the nutrition of the inner layers of the retina, it may be assumed 

 that this is also the primary or sole function of the falciform process, 

 from which nutrients {e.g., glucose) could readily diffuse in all direc- 

 tions through the vitreous, to be absorbed therefrom by the retina. When 

 in later chapters we compare the snakes with the lizards, and the mam- 

 mals with the birds, we shall find in each case an exactly comparable 

 situation: a mutual exclusiveness of two very different mechanisms for 

 the nourishment of the nervous layers of the retina (see pp. 648-58). 



At the distal end of the falciform process lies the 'campanula Halleri' 

 or retractor lentis, with its ectodermal muscular elements and its pig- 

 mented investment derived respectively from the inner and outer layers 

 of the blind retina. Occasionally very small or wholly lacking {e.g., 

 eels, gadids), the muscle when well developed still shows great variabil- 

 ity with regard to size, shape, orientation, presence of tendons (derived 

 from vitreous material) at one or both ends, etc. It pulls directly upon 

 the subspherical lens (which is suspended pendulum-fashion from a 

 dorsonasal suspensory-ligament thickening of vitreous) , drawing it back- 

 ward and temporad. This accommodatory apparatus, like the falciform 

 process, may actually have been invented by the holosteans — possibly 

 even by the chondrosteans {q. v.) ; but it is characteristically teleostean, 

 and no semblance of it occurs in the land vertebrates or in the groups 

 of fishes leading toward them (see next Section). The absence of the 

 falciform process does not affect the presence of the campanula, though 

 this, when a falciform process is present, is usually attached thereto; and 

 the nerve and artery which supply the muscle emerge from the distal 

 part of the process. The two structures sometimes cooperate particularly 

 well, as in the mackerel {Scomber scombrus), where the falciform process 

 lifts completely free of the retina and holds the campanula up to the lens. 



The narrow ciliary zone of the uvea contains a few meridional muscle 

 fibers, simulating closely the 'muscle of Briicke' which accomplishes ac- 

 commodation in the Sauropsida and mammals. In teleosts, the ana- 



