THE ETHMOID BONE 



153 



ossification (Fawcett). 1 The medial joins the lateral pterygoid plate about the sixth month. 

 About the fourth month a center appears for each lingula and speedily joins the rest of the bone. 



The presphenoid is united to the postsphenoid about the eighth month, and at birth the bone 

 is in three pieces (Fig. 148) : a central, consisting of the body and small wings, and two lateral, 

 each comprising a great wing and pterygoid process. In the first year after birth the great wings 

 and body unite, and the small wings extend inward above the anterior part of the body, and, 

 meeting with each other in the middle line, form an elevated smooth surface, termed ihejugum 

 sphenoidale. By the twenty-fifth year the sphenoid and occipital are completely fused. Between 

 the pre- and postsphenoid there are occasionally seen the remains of a canal, the canalis cranio- 

 pharyngeus, through which, in early fetal life, the hypophyseal diverticulum of the buccal ecto- 

 derm is transmitted. 



The sphenoidal sinuses are present as minute cavities at the time of birth (Onodi), but do not 

 attain their full size until after puberty. 



Intrinsic Ligaments of the Sphenoid. The more important of these are: the pterygospinous, 

 stretching between the spina angularis and the lateral pterygoid plate (see cervical fascia) ; the 

 inter clinoid; a fibrous process joining the anterior to the posterior clinoid process; and the 

 caroticoclinoid , connecting the anterior to the middle clinoid process. These ligaments occa- 

 sionally ossify. 



Articulations. The sphenoid articulates with twelve bones: four single, the vomer, ethmoid, 

 frontal, and occipital; and four paired, the parietal, temporal, zygomatic, and palatine. 2 



The Ethmoid Bone (Os Ethmoidale). 



The ethmoid bone is exceedingly light and spongy, and cubical in shape; it 

 is situated at the anterior part of the base of the cranium, between the two orbits, 

 at the roof of the nose, and contributes to each of these cavities. It consists of 

 four parts: a horizontal or cribriform plate, forming part of the base of the cranium; 

 a perpendicular plate, constituting part of the nasal septum; and two lateral masses 

 or labyrinths. 



Cribiform Plate (lamina cribrosa; horizontal lamina}. The cribriform plate 

 (Fig. 149) is received into the ethmoidal notch of the frontal bone and roofs in 

 ihe nasal cavities. Projecting upward from the middle line of this plate is a thick, 

 smooth, triangular process, the crista galli, so called from its resemblance to a 

 cock's comb. The long thin posterior border of the crista galli serves for the 

 attachment of the falx cerebri. 

 ~;ts anterior border, short and 

 i;hick, articulates with the frontal 

 bone, and presents two small pro- 

 .ecting alae, which are received 

 into corresponding depressions in 

 ;he frontal bone and complete 

 ;he foramen cecum. Its sides are 

 smooth, and sometimes bulging 

 Tom the presence of a small air 

 sinus in the interior. On either 

 side of the crista galli, the cribri- 

 form plate is narrow and deeply 

 grooved; it supports the olfactory 

 bulb and is perforated by fora-, 

 tnina for the passage of the olfac- 

 tory nerves. The foramina in the 

 middle of the groove are small 

 and transmit the nerves to the 



roof of the nasal cavity; those at the medial and lateral parts of the groove are 

 larger the former transmit the nerves to the upper part of the nasal septum, 

 the latter those to the superior nasal concha. At the front part of the cribriform 



1 Anatomischer Anzeiger, March, 1905. 

 It also sometimes articulates with the tuberosity of the maxilla (see page 159). 



Perpendicular plate 

 Ala 



Crista galli 



Cribriform plate 



Anterior ethmoidal 

 groove 



Posterior ethmoidal 

 groove 



FIG. 149. Ethmoid bone from above. 



