

THE SPHENOID BONE 147 



stylohyal, and does not appear until after birth. The tympanic ring unites with the squama 

 shortly before birth; the petromastoid part and squama join during the first year, and the tym- 

 panohyal portion of the styloid process about the same time (Figs. 143, 144). The stylohyal 

 does not unite with the rest of the bone until after puberty, and in some skulls never at all. 



The chief subsequent changes in the temporal bone apart from increase in size are: (1) The 

 tympanic ring extends outward and backward to form the tympanic part. This extension does 

 not, however, take place at an equal rate all around the circumference of the ring, but occurs most 

 rapidly on its anterior and posterior portions, and these outgrowths meet and blend, and thus, 

 for a time, there exists in the floor of the meatus a foramen, the foramen of Huschke; this foramen 

 is usually closed about the fifth year, but may persist throughout life. (2) The mandibular fossa 

 is at first extremely shallow, and looks lateralward as well as downward; it becomes deeper and 

 is ultimately directed downward. Its change in direction is accounted for as follows. The part 

 of the squama which forms the fossa lies at first below the level of the zygomatic process. As, 

 however, the base of the skull increases in width, this lower part of the squama is directed hori- 

 zontally inward to contribute to the middle fossa of the skull, and its surfaces therefore come 

 to look upward and downward; the attached portion of the zygomatic process also becomes 

 everted, and projects like a shelf at right angles to the squama. (3) The mastoid portion is at 

 first quite flat, and the stylomastoid foramen and rudimentary styloid process lie immediately 

 behind the tympanic ring. With the development of the air cells the outer part of the mastoid 

 portion grows downward and forward to form the mastoid process, and the styloid process and 

 stylomastoid foramen now come to lie on the under surface. The descent of the foramen is 

 necessarily accompanied by a corresponding lengthening of the facial canal. (4) The downward 

 and forward growth of the mastoid process also pushes forward the tympanic part, so that the 

 portion of it which formed the original floor of the meatus and contained the foramen of Huschke 

 is ultimately found in the anterior wall. (5) The fossa subarcuata becomes filled up and almost 

 obliterated. 



Articulations. The temporal articulates with five bones: occipital, parietal, sphenoid, mandible 

 and zygomatic. 



The Sphenoid Bone (Os Sphenoidale) . 



The sphenoid bone is situated at the base of the skull in front of the temporals 

 and basilar part of the occipital. It somewhat resembles a bat with its wings 

 extended, and is divided into a median portion or body, two great and two small 

 wings extending outward from the sides of the body, and two pterygoid processes 

 which project from it below. 



Body (corpus sphenoidale} . The body, more or less cubical in shape, is hollowed 

 out in its interior to form two large cavities, the sphenoidal air sinuses, which are 

 separated from each other by a septum. 



Surfaces. The superior surface of the body (Fig. 145) presents in front a promi- 

 nent spine, the ethmoidal spine, for articulation with the cribriform plate of the 

 ethmoid; behind this is a smooth surface slightly raised in the middle line, and 

 grooved on either side for the olfactory lobes of the brain. This surface is bounded 

 behind by a ridge, which forms the anterior border of a narrow, transverse groove, 

 the chiasmatic groove (optic groove), above and behind which lies the optic chiasma; 

 the groove ends on either side in the optic foramen, which transmits the optic 

 nerve and ophthalmic artery into the orbital cavity. Behind the chiasmatic 

 groove is an elevation, the tuberculum sellse; and still more posteriorly, a deep 

 depression, the sella turcica, the deepest part of which lodges the hypophysis 

 cerebri and is know r n as the fossa hypophyseos. The anterior boundary of the 

 sella turcica is completed by two small eminences, one on either side, called the 

 middle clinoid processes, while the posterior boundary is formed by a square- 

 shaped plate of bone, the dorsum sellse, ending at its superior angles in two tubercles, 

 the posterior clinoid processes, the size and form of which vary considerably in 

 different individuals. The posterior clinoid processes deepen the sella turcica, 

 and give attachment to the tentorium cerebelli. On either side of the dorsum 

 sellse is a notch for the passage of the abducent nerve, and below the notch a sharp 

 process, the petrosal process, which articulates with the apex of the petrous portion 

 of the temporal bone, and forms the medial boundary of the foramen lacerum. 



