74 SPECIAL ANATOMY OF THE SKELETON 



during the second month, and two more a little lateral to the preceding during the third month. 

 These parts tend to unite, but complete union does not occur until about the fourth month after 

 birth. That part of the tabular portion above the highest curved line is developed by the 

 intrdmembranous method, and may exist in the adult as a separate element, the interparietal 

 bone, or os incae, because of its frequent occurrence in Peruvian skulls. The remainder of 

 the tabular portion and the basilar and two condyloid parts are developed from cartilage. 

 Usually two centres appear in the basilar portion during the sixth fetal week, and by rapid 

 coalescence give the appearance of a single centre. Each condylic portion develops from a 

 single centre that appears at about the end of the eighth fetal week. At about the fourth 

 year the tabular and the two condyloid pieces unite, and about the sixth year the bone 

 consists of a single piece. Between the eighteenth and twenty-fifth years the occipital and 

 sphenoid become united, forming a single bone. 



Articulations. With six bones two parietal, two temporal, sphen6id, and atlas. 



Attachment of Muscles. To twelve pairs to the superior curved line are attached the 

 Occipitofrontalis, Trapezius, and Sternomastoid. To the space between the curved lines, the 

 Complexus, 1 Splenius capitis, and Obliquus capitis superior; to the inferior curved line, and 

 the space between it and the foramen magnum, the Rectus capitis posticus, major and minor; 

 to the transverse process, the Rectus capitis lateralis; and to the basilar process, the Rectus 

 capitis anticus, major and minor, and Superior constrictor of the pharynx. 



The Parietal Bone (Os Parietale). 



The parietal bones are paired bones, and form, by their union, the sides and 

 roof of the cranium proper. Each bone is of an irregular quadrilateral form, 

 and presents for examination two surfaces, four borders, and four angles. 



Surfaces. The external surface (fades parietalis) (Fig. 42) is convex, smooth, and 

 marked about its centre by an eminence called the parietal eminence (tuber parietale), 

 which indicates the point where ossification commenced. Crossing the middle 

 of the bone in an antero-posterior direction are two well-marked curved ridges, 

 the upper and lower temporal ridges (linea temporalis superior et inferior) ; the former 

 gives attachment to the temporal fascia, while the latter indicates the upper limit 

 of the origin of the Temporal muscle. These lines form the temporal crest. 

 Above these ridges the surface of the bone is covered by the aponeurosis of the 

 Occipitofrontalis; below them the bone forms part of the temporal fossa, and 

 affords attachment to the temporal muscle. At the back part of the superior 

 border, close to the sagittal suture, which separates the two parietal bones, is a 

 small foramen (foramen parietale), which transmits the emissary vein of Santorini 

 from the scalp to the superior sagittal sinus. It sometimes also transmits a 

 small branch of the occipital artery. Its existence is not constant, and its size 

 varies considerably. 



The internal or cerebral surface (fades cerebralis) (Fig. 43) is concave, presents 

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

 furrows for the branches of the middle meningeal artery; the latter runs upward 

 and backward from the anterior inferior angle and from the central and posterior 

 part of the lower border of the bone. Sometimes a distinct canal exists for the 

 artery, but it never remains a canal for a long distance. Along the upper margin 

 of the bone is part of a shallow groove, which, when joined to the opposite parietal, 

 forms a channel for the superior sagittal sinus. The elevated edges of the groove 

 afford attachment to the falx cerebri. Near the groove are seen several depres- 

 sions, Pacchionian depressions (foveolae gramdares [Pacchioni]). They are most 

 frequently found in the skulls of old persons, and lodge the arachnoid villi (Pacchi- 

 onian bodies). The internal opening of the parietal foramen is also seen when 

 that aperture exists. On the inner surface of the posterior inferior portion of 

 the bone is a portion of the groove for the lodgement of the lateral sinus. 



1 To these the Biventer cervicis should be added, if it is regarded as a separate muscle. 



