OSTEOLOGY. 29 



part this plate in articulating witli os etlimoides forms 

 foramen orbitale internum. 



OS ETHMOIDES forms a considerable part of the an- 

 terior boundary of the cranium, and consists mainly of a 

 body, from which an ala projects on either side, and a re- 

 markable process projects upwards anteriorly. The body 

 articulates posteriorly with the body of os sphenoides : 

 here it is rounded and solid, but anteriorly it presents two 

 large cavities, ethmoidal sinuses, indirectly continuous with 

 the nasal chamber. The under surface presents centrally 

 a small convex smooth surface, on either side of which is 

 roughened for squamous suture with the vomer. Each of 

 the sides of the body anteriorly is occupied with a squamous 

 surface for os palati, posteriorly by a continuation of that 

 on the posterior part for os sphenoides. The upper surface 

 looks in an upward and slightly backward direction, and 

 is concave. Posteriorly it i:)resents a peculiar depression 

 separated from the rest of this surface by a sharp ridge, 

 optic fossa, from which the two foramina optici pass through 

 the substance of the bone, for passage of the optic nerves, 

 running from the optic decussation, which rests in the fossa. 

 The wings look inward and slightly upwards. Their upper 

 surfaces are completely occupied in formation of the cere- 

 bral cavity ; they therefore present digital impressions. At 

 the line of junction of the u?ider surface with the body of the 

 bone posteriorly is the opening (inferior) of the optic fora- 

 men, and in front of this is a smooth groove, which by union 

 with a similar depression in os sphenoides forms foramen 

 lacerum orbitale. The outer margin of this surface of the ala 

 is mainly occupied by a squamous suture for union with the 

 special groove in os frontis ; and the smooth space be- 

 tween, by some anatomists described as a distinct bone, os 

 planum, serves to form a considerable part of the inner 

 wall of the orbit, fitting into the deep depression extending 

 from the inferior margin of the orbital plate of os frontis. 

 From the antero-superior part of the body crista galli pro- 

 cess runs in an upward direction, to join superiorly the 

 infero- central part of the cranial plates of ossa frontis. 

 Its superior extremity is enlarged, and presents a concave 

 triangular surface for os frontis. Its posterior margin is 

 sharp, smooth on either side and affords attachment 

 to the anterior extremity of falx cerebri. Anteriorly 

 it presents a more or less prominent ridge, extend- 



