236 GENERAL ORNITHOLOGY part ii 



not foreclosed by the frontal bone. You will always find at the 

 back of the orbit, close to the mid-line, and rather low down, 

 the very large holes which transmit the optic nerves from the 

 brain to the eye ; these are the o^iic foramina (any Figs., 2) ; 

 alisphenoid should not extend in front of these oriiices. A 

 little below and behind the optic foramina, and much more 

 laterally, not far from the quadrate itself, is a considerable fora^ 

 men, quite constant, for transmission of the inferior divisions of 

 the fifth {trigeminal or trifacial) nerve. This is the foramen ovale 

 (any Figs., 5) ; it is either in the alisphenoid, or between that bone 

 and the prootic ; it must not be mistaken for one of the several 

 smaller holes, usually seen close about the optic foramen, which 

 transmit the three nerves {oculimotor, pathetic, and abducent) which 

 move the muscles of the eyeball; these holes being collectively 

 about equivalent to the foramen lacerum anterius of human anatomy. 

 Parts about the optic foramen, before and above, are presphenoidal 

 (Figs. 70, 71, ps) and orbitosphenoidal ; but they are obscure to 

 all but the embryologist, and furnish no zoological characters. 



The Ethmoid (Gr. ■^^/io?, ethmos, a sieve; from the way it is 

 perforated in the human species; Fig. 62) is the bone of the mid- 

 line of the skull, in front of the sphenoidal elements and below the 

 frontal ; it is in special relation with the olfactory nervous apparatus, 

 or sense of smell. This is not an easy bone to " get the hang of " 

 in birds. Referring to Figs. 66, 68, eth, the student will see in the 

 early embryo a high thin plate of cartilage, the mesethmoid car- 

 tilage, which is developing lateral processes to form the convoluted 

 walls of the nasal passages. By the uprising and forth-growing of 

 the prenasal cartilage, the mesethmoidal plate is tilted backward, 

 as it were, under the frontal. Next, by absorption of tissue just 

 opposite the future craniofacial suture, the plate is nicked apart, 

 the portion in front of the nick elaborating the nasal chambers, 

 which usually remain cartilaginous, and the portion behind this 

 nick becoming the permanent plate, Fig. 70, eth, pe, to which the 

 name mesethmoid or mid-ethmoid is more strictly applicable. Prac- 

 tically, a bird's ethmoid is chiefly the interorbital septum, in vertical 

 mid-line between the orbits, with such flange-like processes or lateral 

 plates as may be developed to form an orbitonasal septum separating 

 the eye-socket from the nose-chamber. In general, the permanent 

 ethmoidal plate becomes nearly coincident with this orbital wall, 

 and pretty well cut ofi' from the osseous or cartilaginous develop- 

 ments, when any, in the nasal cavities. It is then fairly under 

 cover of the frontal, with which, as with the sphenoidal elements 

 posteriorly, it becomes completely fused. When this interorbital 

 septum is fully developed, it completely divides the right and left 

 orbital cavities, and its lower horizontal border, fused with the 



