TEMPORAL BOXE. G9 



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 (pao-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, being sometimes 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 pro- 

 cess, the size and form of which vary somewhat in different individuals. This 

 process serves for the attachment of the Sterno-mastoid, 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 deeply 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 tym- 

 panum ; are lined by a prolongation 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 promi- 

 nence 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 (rtltpos, a rock), 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 forwards, inwards, 

 and a little downwards. It presents for examination a base, an apex, three siir- 

 faces, 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 their divergence, which brings into view the oval expanded orifice of a canal 

 leading into the tympanum, the meatus auditorius externus. This canal is situated 

 between the mastoid process and the posterior and middle roots of the zygoma ; 

 its upper margin is smooth and rounded, but the greater part of its circumference 

 is surrounded by a curved plate of bone, the x auditory process, the free margin of 

 which is thick and rough for the attachment of the cartilage of the external ear. 



The apex of the petrous portion, rough and uneven, is received into the angular 

 interval between the spinous process of the sphenoid, and the basilar process of 

 the occipital ; it presents the anterior orifice of the carotid canal, and forms the 

 posterior and external boundary of the foramen lacerum medium. 



The anterior surface of the petrous portion (fig. 31) forms the posterior 

 boundary of the middle fossa of the skull. This surface is continuous with the 

 squamous portion, to which it is united by a suture, the temporal future, the 

 remains of which are distinct at a late period of life. This surface presents six 

 points for examination. 1. An eminence near the centre which indicates the 

 situation of the superior semicircular canal. 2. On the outer side of this eminence 

 is a depression, indicating the position of the tympanum, the layer of bone which 

 separates the tympanum from the cranial cavity being extremely thin. 3. A 

 shallow groove, sometimes double, leading backwards to an oblique opening, the 

 hiatus Fallopii, for the passage of the petrosal branch of the Yidian nerve. 4. 

 A smaller opening, occasionally seen external to the latter for the passage of the 



