The Bones of the Skull, 91 



vening between the tympanic membrane and the opening to the internal 

 ear as represented by the vestibular fenestra. They occupy the dorsal 

 angle of the triangular area already described, and lie immediately above 

 the promontory. The malleus is the lateral element. The main por- 

 tion, termed the head, is concealed by the projecting edge of the external 

 acoustic meatus. It bears a stout vertical process, the manubrium 

 mallei, which in the natural condition lies in contact with the tympanic 

 membrane. The incus is the intermediate element; it is directly 

 articulated with the malleus, and bears a downwardly-directed long limb 

 (crus longum), for articulation with the minute head of the stapes. 

 The latter element is a small, stirrup-shaped bone, occupying an almost 

 transverse position, and articulated at its base with the margin of 

 the vestibular fenestra. 



7. THE INTERPARIETAL BONE. 



The interparietal (os interparietale) is a small, lozenge-shaped element, 

 surrounded by the two parietal bones and the supraoccipital. It is the 

 first of the membrane roofing elements of the cranium proceeding for- 

 ward from the supraoccipital, and in the rabbit's skull is not fused with 

 the occipital segment. 



8. THE PARIETAL BONE. 



The parietal bone (os parietale) is a characteristic roofing bone 

 covering a large portion of the middle cranial fossa It is somewhat 

 rectangular in shape, and is connected by serrate sutures with the sur- 

 rounding elements and with its fellow of the opposite side, the sutures 

 producing a characteristic pattern on the external surface of the skull. 

 The sutures are medial, anterior, lateral, and posterior in position, and 

 are designated respectively as sagittal, coronal, squamosal, and lamb- 

 doidal. The posterolateral angle of the bone is produced ventrally into 

 a long, curved squamous process (processus squamosus), which lies in 

 the angle formed by the tentorium cerebelli and the lateral wall of the 

 middle cranial fossa. It is not exposed to the external surface of the 

 skull. 



9. THE FRONTAL BONE. 



The frontal bone (os frontale) is a paired element, lying directly in 

 front of the parietal, and forming with its fellow of the opposite side the 

 anterior portion of the roof of the cranial cavity and also a considerable 

 portion of its lateral, orbital wall. Unlike the condition in the human 

 skull, the two bones are separate throughout life, so that there is a per- 

 manent frontal suture. Each consists of a frontal portion (pars frontalis), 

 the external or dorsal surface of which continues that of the parietal, 

 and of an orbital portion (pars orbitalis). enclosing the dorsal portion of 

 the orbit. The two parts are connected at the supraorbital border, with 

 which is also connected the base of the divided supraorbital process. 

 The anterior end of the frontal portion is deeply notched where it comes 

 in contact with the nasal and premaxillary bones. Two processes are 



