92 A MANUAL OF ANATOMY 



aspect of that eminence, which may even extend through it and give rise 

 to a small foramen on its upper surface. 



At birth the bone is composed of three parts — a central, representing the 

 presphenoid and postsphenoid portions of the body, the former bearing 

 the small wings ; and two lateral, each of which represents a great wing bearing 

 a pterygoid process. In the first year the lingula joins the great wing, and 

 the wing and body unite. About the same time the small wings come together 

 and blend over the anterior part of the upper surface of the presphenoid 

 portion of the .body, where they give rise to a smooth, elevated, flat platform, 

 called the jugum sphenoidale. 



In foetal hfe a canal, called the cranio-pharyngeal canal, leads downwards 

 from the sella turcica into the body, and contains a process of the dura 

 mater. This canal is the remains of a cleft originally present in the base of 

 the skull, through which a diverticulum of the buccal epiblast, known as the 

 pouch ol Rathke, originally passed upwards to form the anterior lobe of the 

 pituitary body. 



The sphenoidal air sinuses do not appear until after the seventh year. 



In some animals the presphenoid and postsphenoid portions remain 

 permanently separate, and the internal pterygoid plates form the pterygoid 

 bones. 



The sphenoidal turbinate bones commence to ossify in the fifth month of 

 intra-uterine life. At birth each partially envelops a small extension of the 

 nasal mucous membrane, and by the third year it has surrounded it in the 

 form of a bony capsule, except anteriorly, where an opening, called the sphe- 

 noidal foramen, is left. Subsequently a portion of this capsule becomes 

 absorbed, and its place is taken by the presphenoid, which latter, after the 

 seventh year, is gradually invaded by the original extension of the nasal 

 mucous membrane. The sphenoidal spongy bones become ankylosed to the 

 ethmoid about the fourth year, and are sometimes regarded as belonging 

 to that bone. By the twelfth year they have become united to the sphenoid, 

 and also to the palate bones. 



The Ethmoid Bone. 



The ethmoid bone is situated at the anterior part of the 

 base of the skull, where it lies in the middle line in front of the 

 sphenoid. A portion of it occupies the ethmoidal notch between 

 the orbital plates of the frontal, whence the greater part of the 

 bone projects downwards to take part in the formation of the 

 orbits and nasal fossae. The only portions of the bone visible in 

 the interior of the base are the cribriform plate and crista galli. 

 It is irregularly cubical, its long axis being directed from before 

 backwards, and it is remarkable for its lightness, which is due to 

 the great number of enclosed air cells, these being surrounded by 

 very thin plates of bone. It is composed of four parts, namely, 

 a cribriform plate, a perpendicular plate, and two lateral masses. 



The cribriform plate connects the upper borders of the lateral 

 masses, and enters into the formation of the middle division of the 

 anterior cranial fossa, where it occupies the ethmoidal notch of the 

 frontal bone. In the middle line anteriorly it presents an upward 

 extension of the perpendicular plate, called the crista galli. This 

 is a stout, triangular, laterally-compressed process, which presents 

 a smooth, sloping posterior border, for the falx cerebri. The 

 anterior border, short and vertical, is somewhat narrow above, 

 but soon expands into two alar processes, for the frontal bone, 



