THE TEMPORAL BONES.

Borders.-The superior border is thin, bevelled at the expense of the internal surface, so as .to overlap the lower border of the parietal bone, forming the squam¬ous suture. The anterior inferior border is thick, serrated, and bevelled, alter¬nately at the expense of the inner and outer surfaces, for articulation with the great wing of the sphenoid.

. The Mastoid Portion  is situated at the posterior part of the bone; its outer surface is rough, and gives attachment to the Occipito-frontalis and Retrahens aurem muscles. It is 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 from the occipital to supply the dura mater. 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-mastoid, ~Jllenius capitis, and 'rl'achelo-mastoid muscles. On the inner side of the mastoid process is a deep groove, the digastric fossa, for the attachment of the Diga"tric muscle; and, running parallel with it, but more in¬ternal, the occipital gronee. which lodges the occipital artery. The internal surface of the mastoid portion presents a deep, curved groove

. The groove for the lateral sinus is separated from the innermost of the mastoid air-cells by only a thin lamina of bone, and even this may be partly deficient. 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, which exhibit the greatest possible variety us to their size and number.