34 TEMPORAL BOXE. 



opening, the carotid foramen, the commencement of the carotid canal, 

 which lodges the internal carotid artery and the carotid plexus. And 

 between the stylo-mastoid and carotid foramen, in the posterior border, 

 is an irregular excavation forming part of the jugular fossa for the 

 commencement of the internal jugular vein. The proportion of the 

 jugular fossa formed by the petrous portion of the temporal bone is 

 very different in different bones ; but in all, the fossa presents a vertical 

 ridge to its inner side, which cuts off a small portion from the rest. 

 The upper part of this ridge forms a spinous projection, which is 

 called the jttgular process, the groove to the inner side of the ridge 

 lodges the eighth pair of nerves, and the lower part of the ridge is the 

 septum of division between the jugular fossa and the carotid foramen. 

 Upon this portion of the ridge near the posterior margin of the carotid 

 foramen is a small opening leading into a canal, which transmits the 

 tympanic branch of the glossopharyngeal nerve (Jacobson's nerve). 

 Between the jugular fossa and the stylo-mastoid foramen is another 

 small opening leading into the canal for the passage of the tympanic 

 branch of the pneumogastric nerve. 



Borders. The superior border is sharp, and gives attachment to the 

 tentorium cerebelli. It is grooved for the superior petrosal sinus, and 

 near its extremity is marked by a smooth notch upon which reclines 

 the fifth nerve. 



The anterior border is grooved for the Eustachian tube, and forms 

 the posterior boundary of the foramen lacerum basis cranii ; by its 

 sharp extremity it gives attachment to the tensor tympani and levator 

 palati muscles. The posterior border is grooved for the inferior petro- 

 sal sinus, and excavated for the jugular fossa; it forms the anterior 

 boundary of the foramen lacerum posterius. 



Development. By five centres ; one for the squamous portion, one 

 for the mastoid process, one for the petrous portion, one for the audi- 

 tory process, which in the foetus is a mere bony ring, incomplete supe- 

 riorly, and serving for the attachment of the membrana tympani, 

 annulus membranes tympani ; and one for the styloid process. Ossifi- 

 cation occurs in these pieces in the following order : in the squamous 

 portion immediately after the vertebrae, then in the petrous, tympanic, 

 mastoid, and styloid. The tympanic ring is united by its extremities 

 to the squamous portion during the last month of intrauterine life ; the 

 squamous, petrous and mastoid portions are consolidated during the 

 first year: and the styloid some years after birth. It not unfrequently 

 happens that the latter remains permanently separate, or is prolonged by 

 a series of pieces to the os hyoides, and so completes the hyoid arch. 

 The subsequent changes in the bone are the increase of size of the 

 glenoid fossa, the growth of the meatus auditorius externus, the level- 

 ling of the surfaces of the petrous portion and the development of 

 mastoid cells. Traces of the union of the petrous with the squamous 

 portion of the bone are usually perceptible in the adult. 



Articulations. With five bones ; occipital, parietal, sphenoid, in- 

 ferior maxillary and malar. 



