TEMPORAL BONE. 51 



''or?/ meatus a short canal through which the auditory and 

 facial motor nerves pass, and which is divided by a thin plate 

 into two parts, one leading by small foramina to the internal ear, 

 the other being the origin of the aqueduct of Fallopius, which, 

 after taking a spiral course through the bone, terminates externally 

 bv the stylo-mastoid foramen. A narrow slit, the aqueduct of the 

 ule : near the meatus, transmits a small vein and artery, and 

 lodges a process of dura mater. 



The four surfaces are separated by four borders or angles, two 

 of which deserve notice, the first, separating the external from 

 the posterior surface, is thick and rough, and forms the mastoid 

 ridge, which is continuous with the occipital ridge above, and, 

 has the posterior mastoid process in front ; the mastoid fissure 

 runs across this angle, passing upwards under the squamosal bone 

 to terminate in the parieto-temporal conduit. The second, 

 separating the anterior from the internal surface, marks the 

 division between the cavities of the cerebrum and cerebellum, and 

 has a process of dura mater attached. 



The summit of the bone is pointed, and articulates with the 

 occipital ; the base is very irregular in outline, and presents a 

 large opening, the external auditory meatus, which leads to 

 the middle ear, and is surrounded by an oval rim of bone, 

 which is notched in the middle, and gives attachment to the 

 cartilages of the ear. Below this opening is the hyoid process, 

 to which is attached the cornu of the hyoid bone, and between 

 which and the mastoid process is the stylo-mastoid foramen, the 

 external opening of the aqueduct of Fallopius. The rounded 

 part immediately below the hyoid process is the mastoid pro- 

 tuberance or auditory bulla, which is little developed in the 

 horse, containing the mastoid cells ; before, and projecting 

 forwards, is the long and slender styloid process, which gives 

 attachment to muscles, the Glaserian fissure (styloid foi'amen) 

 for the corda tympani nerve being above it, and the opening for 

 the Eustachian tube below and inside. More internally, a well- 

 marked ridge forms the external boundary of the foramen laceruiu 

 basis cranii. 



The petrosal bone articulates with the occipital, parietal, and 

 squamosal bones, and with the hyoidean cornu. 



