154 



THE SKELETON. 



Attachment of Muscles. The Temporal, External Pterygoid, Internal Ptery- 

 gold, Superior Constrictor, Tensor Palati, Laxator Tyrapani, Levator Palpebrae, 

 Obliquus Superior, Superior Kectus, Internal Eectus, Inferior Eectus, External 

 Eectus. 



THE ETHMOID BONE. 



The Ethmoid (Jfyoj, sieve) is an exceedingly light spongy bone, of a cubical 

 form, situated at the anterior part of the base of the cranium, between the two 



orbits, at the root of the nose, 

 Fig. 105.-Ethmoid Bone. Outer Surface of Right Lateral and contributing to form each 



of these cavities. It consists 

 of three parts : a horizontal 

 plate, which forms part of the 

 base of the cranium ; a per- 

 pendicular plate, which forms 

 part of the septum nasi ; and 

 two lateral masses of cells. 



The Horizontal or Cribriform 

 Plate (Fig. 105) forms part of 

 the anterior fossa of the base 

 of the skull, and is received 

 into the ethmoid notch of the 

 frontal bone between the two 

 orbital plates. Projecting up- 

 wards from the middle line of 

 this plate, is a thick, smooth, 

 triangular process of bone, 

 the crista galli, so called from 



its resemblance to a cock's comb. Its base joins the cribriform plate. Its 

 posterior border, long, thin, and slightly curved, serves for the attachment 

 of the falx cerebri. Its anterior border, short and thick, articulates with the 

 frontal bone, and presents two small projecting alae, which are received into 

 corresponding depressions in the frontal, completing the foramen ccecum behind. 



Its sides are smooth, 

 and sometimes bulging ; 

 in which case it is found 

 to inclose a small sinus. 

 On each side of the 

 crista galli, the cribri- 

 form plate is narrow, 

 and deeply grooved, to 

 support the bulb of the 

 olfactory nerve, and 

 perforated by foramina 

 for the passage of its 

 filaments. These fora- 

 arranged 



iik inf. 



Fig. 106. Perpendicular Plate of Ethmoid (enlarged), shown by 

 removing the right Lateral Mass. 







mina are arranged m 

 three rows: the inner- 

 most, which are the 

 largest and least nume- 

 rous, are lost in grooves 

 on the upper part of the 

 septum; the foramina 

 of the outer row are 



continued on to the surface of the upper spongy bone. The foramina of the 

 middle row are the smallest; they perforate the bone, and transmit nerves to 



