68 TEMPORAL BONE. 



large oval opening, the carotid foramen, the commencement of the caiotul 

 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 differen 

 in different bones ; but in all, the fossa presents a vertical ridge on it 

 inner side, which cuts off a small portion from the rest. The upper par 

 of this ridge forms a spinous projection, which is called the jugular 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 jugu- 

 lar 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 (Jacobson's nerve) of the 

 glossopharyngeal nerve. Between the jugular fossa and the stylo-mastoid 

 foramen is another small opening leading into the canal for the passage of 

 the auricular 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 petrosal sinus, 

 and excavated for the jugular fossa ; it forms the anterior boundary of the 

 foramen lacerum posterius. 



Development. By Jive centres : one for the squamous portion, one for 

 the mastoid process, one for the petrous portion, one for the auditory pro- 

 cess, which in the fretus is a mere bony ring, incomplete superiorly, and 

 serving for the attachment of the membrana tympani, annulus membrana 

 tympani ; and one for the styloid process. Ossification 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 intra-uterine 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 perma- 

 nently 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 levelling 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 Jive bones : occipital, parietal, sphenoid, inferior 

 maxillary, and malar. 



Attachment of Muscles. To fourteen: by the squamous portion, to the 

 temporal ; by the zygoma, to the masseter ; by the mastoid portion, to 

 the occipito-frontalis, splenius capitis, sterno-mastoid, trachelo-mastoid, 

 digastricus and retrahens aurem; by the styloid process, to the stylo- 

 r >naryngeus, stylo-hyoideus, stylo-glossus, and two ligaments, the stylo- 



