. On the anatomy of vertebrates [electronic resource] . fications of the homo-logue of the arteria centralis retinas: this is not continued as asimple branch from its origin to the marsupium; but, imme-diately before penetrating the coats of the eye, it breaks intonumerous subdivisions, the aggregate of which is greater than thetrunk whence they proceed, and these again unite, forming aplexus, s, fig. 59, close to the external side of the optic artery of the marsupium proceeds from this plexus, and runsalong the base of the folds, giving off at right angles a branch toeach fold, whic
. On the anatomy of vertebrates [electronic resource] . fications of the homo-logue of the arteria centralis retinas: this is not continued as asimple branch from its origin to the marsupium; but, imme-diately before penetrating the coats of the eye, it breaks intonumerous subdivisions, the aggregate of which is greater than thetrunk whence they proceed, and these again unite, forming aplexus, s, fig. 59, close to the external side of the optic artery of the marsupium proceeds from this plexus, and runsalong the base of the folds, giving off at right angles a branch toeach fold, which in like manner sends off smaller ramuli, fig. plexus at the origin of the marsupial artery serves as areservoir for supplying the blood required for the occasional fullinjection of the marsupium; and a similar but larger plexus, , 4, is formed at the origins of the ciliary arteries which supplythe erectile tissue of the ciliary processes and iris. The vitreous humour presents few peculiarities worthy of note; 142 ANATOMY OF compared with the aqueous humour, it is proportionally less inquantity than in the eyes of Mammals. The outer capsule formedby the hyaloid membrane is stronger, and can be more easilyseparated from the humour. The eyeball is moved in Birds by four straight and two ob-lique muscles. The Recti muscles arise from the circumferenceof the optic foramen, and expand, as they pass forward, to beinserted into the soft middle part of the sclerotic. We have notbeen able to trace their insertion dis-tinctly to the osseous circle; their apo-neurosis cannot be reflected forward fromthe sclerotica without lacerating thatmembrane. The Obliqui both arise very near to-gether from the anterior parietes of theorbit, and go to be inserted, the one intothe upper, the other into the lower partof the globe of the eye; the superiorobliquus does not pass through a pulley,as in Mammalia. All the muscles areproportionally short in this class, but especially
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