TEMPORAL BONES. 145 



recent state with cartilage, and articulates with the condyle of the lower jaw. 

 This part of the glenoid fossa is separated from the auditory process by a small 

 tubercle, the post- glenoid process, the representative of a prominent tubercle 

 which, in some of the mammalia, descends behind the condyle of the jaw, and 

 prevents it being displaced backwards during mastication (Humphry). The 

 posterior part of the glenoid fossa is formed chiefly by the vaginal process of 

 the petrous portion, and lodges part of the parotid gland. The Glaserian 

 fissure, which leads into the tympanum, lodges the processus gracilis of the 

 malleus, and transmits the Laxator Tympani muscle and the tympanic branch 

 of the internal maxillary artery. The chorda tympani nerve passes through a 

 separate canal parallel to the Glaserian fissure (canal of Huguier), on the outer 

 side of the Eustachian tube, in the retiring angle between the squamous and 

 petrous portions of the temporal bone. 



The internal surface of the squamous portion (Fig. 98) is concave, presents 

 numerous eminences and depressions for the convolutions of the cerebrum, and 

 two well-marked grooves for the branches of the middle meningeal artery. 



Borders. The superior border is thin, bevelled at the expense of the inter- 

 nal surface, so as to overlap the lower border of the parietal bone, forming the 

 squamous suture. The anterior inferior border is thick, serrated, and bevelled, 

 alternately at the expense of the inner and outer surfaces, for articulation with 

 the great wing of the sphenoid. 



The Mastoid Portion (paa-eos, a nipple or teat] is situated at the posterior part 

 of the bone ; its outer surface is rough, and perforated by numerous foramina : 

 one of these, of large size, situated at the posterior border of the bone, is 

 termed the mastoid foramen ; it transmits a vein to the lateral sinus and a small 

 artery. The position and size of this foramen are very variable. It is not 

 always present : sometimes it is situated in the occipital bone, or in the suture 

 between the temporal and the occipital. The mastoid portion is continued 

 below into a conical projection, the mastoid process, the size and form of which 

 vary somewhat. This process serves for the attachment of the Sterno-mas- 

 toid, Splenius Capitis, and Trachelo-mastoid muscles. On the inner side of 

 the mastoid process is a deep groove, the digastric fossa, for the attachment of 

 the Digastric muscle; and running parallel with it, but more internal, the 

 occipital groove, which lodges the occipital artery. The internal surface of 

 the mastoid portion presents a deep curved groove, which lodges part of the 

 lateral sinus ; and into it may be seen opening the mastoid foramen. A section 

 of the mastoid process shows it to be hollowed out into a number of cellular 

 spaces, communicating with each other, called the mastoid cells ; they open by 

 a single or double orifice into the back of the tympanum; are lined by a pro- 

 longation of its lining membrane ; and, probably, form some secondary part 

 of the organ of hearing. The mastoid cells, like the other sinuses of the 

 cranium, are not developed until after puberty ; hence the prominence of this 

 process in the adult. 



Borders. The superior border of the mastoid portion is broad and rough, its 

 serrated edge sloping outwards, for articulation with the posterior inferior 

 angle of the parietal bone. The posterior border, also uneven and serrated, 

 articulates with the inferior border of the occipital bone between its lateral 

 angle and jugular process. 



The Petrous Portion (rts'fpoj, a stone), so named from its extreme density and 

 hardness, is a pyramidal process of bone, wedged in at the base of the skull 

 between the sphenoid and occipital bones. Its direction from without is in- 

 wards, forwards, and a little downwards. It presents for examination a base, 

 an apex, three surfaces, and three borders ; and contains, in its interior, the 

 essential parts of the organ of hearing. The base is applied against the internal 

 surface of the squamous and mastoid portions, its upper half being concealed ; 

 but its lower half is exposed by the divergence of those two portions of the 



