Chap. V] 



THE SKELETON 



57 



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 por- 



DIGASTRIC 

 FOSSA 



Fig. 27. — The Right Temporal Bone. Outer surface. The dotted lines 

 indicate the lines of suture between squamous, mastoid, and petrous portions. 

 (Gerrish.) 



tions is a socket, called the glenoid fossa, for the reception of the 

 condyle of the lower jaw. 



Ethmoid bone. — -It is an exceedingly light cancellous bone 

 that forms part of the orbits, nasal fossae, and base of the cranium. 

 It consists of a horizontal plate, a verti- 

 cal plate, and two lateral masses. The 

 horizontal plate forms the roof of the 

 nasal fossae, and also closes the anterior 

 part of the base of the cranium. It is 

 pierced by numerous foramina or holes, 

 through which the nerves conveying the 

 sense of smell pass. Descending from 

 the horizontal plate is the vertical plate 

 which helps to form the nasal septum, 

 and on either side the lateral masses 

 help to form the side walls of the nasal 



fossse. The lateral masses contain a Fig. 28. - Ethmoid Bone. 



Seen fromunder surface. 2,criD- 

 number of thin-walled cavities called riform or perforated plate. 



