36 



ANATOMY FOR NURSES. 



[Chap. IV. 



dense portion, called petrous; a thin and expanded scale-like 

 portion, called squamous ; and a mastoid portion, which is per- 

 forated by numerous holes and contains a number of sinuses or 



Fig. 29. — Sphenoid Bone, a, greater wing; b, lesser wing. 



air spaces. The internal ear, the essential part of the organ 

 of hearing, is contained in a series of cavities, channelled out of 

 the substance of the petrous portion. Between the squamous 



and petrous portions is a socket for 

 the reception of the condyle of the 

 lower jaw. 



The sphenoid bone {sphen, a wedge) 

 is situated at the anterior part of the 

 base of the skull, articulating with all 

 the other cranial bones, which it binds 

 firmly and solidly together. In form 

 it somewhat resembles a bat with ex- 

 tended wings. 



The ethmoid bone is an exceedingly 

 light, spongy bone, placed between the 

 two orbits and at the root of the nose, 

 contributing to form a part of each of 

 these cavities. The portion of the bone 

 situated at the back of the nose, which forms the roof of the 

 nasal fossae and also closes the anterior part of the base of 

 the skull cavity, is pierced by numerous holes, through which 



Fig. 30. — E j;hmoii> Bone 

 Posterior surface. 2, cribri 

 form, or perforated plate. 



