54 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 57. — The right temporal bone seen from the cerebral surface and from the apex of the petrous 

 portion (\). 



Fig. 58. — The right temporal bone seen from below (}). 



Fig. 59. — The right temporal bone of a new-born child (f). 



Fig. 60. — The left temporal bone of a four-year-old child (f). 



In Figs. 59 and 60 the squamous portion is green, the petrous and mastoid portions yellow, and the tympanic 

 portion white. 



and 63), and these may become so large, particularly in later life, that they markedly attenuate 

 the bony tissue and even expand it. They are connected with the cavity of the middle ear, 

 the tympanic antrum. 



The mastoid process is one of the chief points for muscular attachment which the skull possesses, and it receives 

 the insertions of the sternocleidomastoid and of portions of the splenius capitis and of the longissimus capitis muscle's. 

 The mastoid notch gives origin to the posterior belly of the digastric muscle. Several openings may be present in the 

 vicinity of the mastoid foramen. The depression beside the suprameatal spine and above the mastoid process is also 

 called the mastoid fossa. 



THE PETROUS PORTION. 



The petrous portion or pyramid of the temporal bone (Figs. 56 to 58) forms a portion of 

 both the external and internal surfaces of the base of the skull (see Figs. 42 and 43). It has 

 the shape of a three-sided horizontal pyramid, two of the surfaces being directed toward the 

 cranial cavity and one externally, and the axis of the pyramid passing obliquely from behind 

 forward and from without inward. There may consequently be distinguished internally an 

 anterior surface and a posterior surface, and externally an inferior surface, and there is a superior, 

 an anterior, and a posterior border. The superior border separates the two cerebral surfaces; 

 the two remaining borders separate the cerebral surfaces from the external one. 



The petrous portion in the adult is directly continuous externally with the tympanic portion, 

 and its internal anterior surface borders upon the squamous portion (petrosquamosal fissure, 

 see page 53) and upon the sphenoid bone (sphenopetrosal fissure). The base and a part of the 

 posterior margin is continuous with the mastoid portion even during early fetal life, and the 

 anterior portion of the posterior margin articulates with the lateral portion of the occipital bone 

 (petro-occipital fissure). The apex of the petrous portion projects into the foramen lacerum 

 (see Fig. 42), in the space between the sphenoid and the occipital bones. 



The anterior cerebral surface forms a portion of the floor of the middle cerebral fossa, 

 and presents a flattened projection lying at right angles to the axis of the pyramid and known 

 as the arcuate eminence (Fig. 56), because it overlies the semicircular canal of the internal ear 

 which is embedded in the petrous portion of the bone. Upon this anterior surface there is 

 also a small slit-like orifice, the hiatus Fallopii {hiatus canalis facialis) (Fig. 57), from which 

 the great superficial petrosal nerve passes toward the apex of the pyramid and to the foramen 

 lacerum in a groove (Fig. 57). 



External and anterior to the hiatus canalis facialis there is a second smaller opening, the 

 superior aperture 0} the tympanic canaliculus (Fig. 57), the place of exit of the lesser superficial 

 petrosal nerve, which also passes forward to the region of the foramen lacerum in a groove. 



