24 ANATOMY OF THE HORSE. 



by the anterior mastoid process, which descends in the form of a 

 cone, serves to strengthen and hmit the motion of the articula- 

 tion, and gives attachment to part of the levator humeri. Be- 

 hind the root of the posterior articular process is the mastoid 

 foramen, for transmitting blood-vessels to and from the temporal 

 fossa. Farther backward is the orifice of the temporal conduit, 

 which gives passage to a vein. 



The Internal or Cerebral. Surface is concave, to contain and 

 support the posterior lobes of the cerebrum by which it is 

 imprinted, and displays several arborescent grooves, mark- 

 ing the course of the posterior arterial ramifications of the 

 dura mater. 



The Circumference, ovoid and elongated po&teriorly, is denti- 

 culated and squamous, excepting at the posterior part, where it 

 is joined to the petrous portion. The superior part lies upon 

 the side of the parietal bone, and behind that upon a very small 

 proportion of the occipital ; in front, it lies upon the frontal, 

 and below upon the sphenoid. 



THE PETROUS PORTION. (PARS PETROSA.) 



Is important from lodging the internal organ of hearing. It 

 presents two parts perfectly distinct from each other in structure, 

 position, and use ; one is external or mastoideal, the other inter- 

 nal or cerebral. From the former projects downwards the posterior 

 mammiform or mastoid process, in which is distinguished a base 

 inferiorly, and a crest superiorly : the latter joins the occipital 

 crest. Below and rather towards the front of the process is the 

 external orifice of the aqueduct of the vestibule ; directly in 

 front of the process is the external auditory meatus and 

 foramen, surrounded by a jutting oval rim, notched in the 

 middle, which constitutes the auditory process ; to the inner 

 side of the meatus is the hyoideal process, surrounded by 

 a prominent circular edge, with which is connected the 

 OS hyoides. Behind the meatus is a general protuberance, 

 spheroid without, cavernous within, bottomed by a thin bony 

 shell, within which are inclosed the mastoid cells ; proceeding 

 from the inferior, anterior, and inner part of the bone is the 

 styloid process; at whose root are two openings, one below for 

 the Eustachian tube, the other above, the styloid foramen, for 

 the chorda tympani. 



The Internal Part exhibits three surfaces for consideration. 

 One superior and posterior, smooth though uneven, is excavated 

 for adaptation to the side of the cerebellum, by which it is im- 

 pressed in places ; it has a prominence running across its 



