THE ETHMOID BONE. 47 



muscle. The internal pier ygoid plate is longer and narrower than the external, and 

 is prolonged below into the slender hook-like or hamular process, round which in a 

 groove plays the tendon of the tensor palati muscle. Above, the internal plate turns 

 inwards beneath the body, from which its extremity remains distinct as a slightly 

 raised edge, known as the vaginal process, which articulates with the everted margin 

 of the vomer ; externally to this it is marked by a small groove, which contributes 

 with the palate bone to form the pterygo-palatine canal. Posteriorly, the internal 

 pterygoid plate forms at its base a small blunt prominence, the pterygoid tubercle, to 

 the inner side of and below the orifice of the Vidian canal ; between this and the 

 pterygoid fossa is a slight depression, called the navicular or scaphoid fossa, occupied 

 by the attachment of the tensor palati muscle ; and lower down, on the hinder 

 margin of the plate, is a projection which supports the cartilage of the Eustachian 

 tube. The interval between the lower ends of the pterygoid plates, pterygoid notch, 

 is occupied by the pyramidal process of the palate bone. 



Fissures and foramina. Each lateral half of the bone presents a fissure, four 

 foramina, and a canal. The sphenoidal fissure is the obliquely placed elongated 

 interval between the great and small wings, closed externally by the frontal bone ; 

 it opens into the orbit, and transmits the third, fourth, and sixth nerves, the 

 ophthalmic division of the fifth nerve, and the ophthalmic veins. Above and to the 

 inside of the sphenoidal fissure is the toptic foramen, which is inclined outwards and 

 forwards from the side of the olivary eminence, pierces the base of the small wing, 

 and transmits the optic nerve and the ophthalmic artery. The foramen rotundum is 

 directed forwards through the great wing, below the sphenoidal fissure ; it opens 

 immediately below the level of the orbit, and transmits the superior maxillary nerve. 

 The foramen ovale is large, and placed behind and a little external to the foramen 

 rotundum, near the posterior margin of the great wing ; it is directed downwards, 

 and transmits the inferior maxillary nerve and small meningeal artery. The 

 foramen spinosum is a small foramen piercing the great wing, near its posterior 

 angle, and transmits the large middle meningeal vessels. 



The Vidian or pterygoid canal pierces the bone in the sagittal direction at the 

 base of the internal pterygoid plate. It opens anteriorly into the spheno-maxillary 

 fossa, and posteriorly into the foramen lacerum ; and through it pass the Vidian 

 nerve and vessels. 



Varieties. A small tubercle is often seen on each side in front of the pituitary fossa, at 

 the base of the olivary eminence, and immediately internal to the last part of the carotid 

 groove ; this is known as the middle clinoid process, and is sometimes connected by a spiculum 

 of bone to the anterior clinoid process, forming a carotico-clinoid foramen. Less frequently 

 the anterior and posterior clinoid processes are similarly united. There are normally fibrous 

 bands, interclinoid ligaments, beneath the dura mater in these situations (Gruber). In some 

 cases a superior petrosal process projects from the lateral margin of the dorsum sellas (fig. 69), 

 giving attachment to the petro-sphenoidal ligament (p. 43). The outer pterygoid plate may 

 be connected by a bridge of bone or of ligament (pterygo-spinous) with the spinous process. 

 The foramen ovale and foramen spinosum are frequently incomplete at the posterior margin 

 of the bone. The name of foramen of Vesalius is given to an aperture sometimes present on 

 the inner side of the foramen ovale : it gives passage to an emissary vein. At the base of the 

 spinous process, to the inner side of the foramen spinosum, there is occasionally a minute canal 

 (canaliculus innominatus Arnold) transmitting the small superficial petrosal nerve. 



THE ETHMOID BONE. 



The ethmoid, or sieve-like bone, projects downwards from between the orbital 

 plates of the frontal bone, and enters into the formation of the cranium, the orbits, 

 and the nasal fossa?. It is of a cuboid figure, and exceedingly light for its size, 

 being composed of very thin plates of bone surrounding in great part irregular cells. 



