THE BOXES OF THE SKULL 189 



cerebelli and the lateral wall of the middle cranial fossa. It is not 

 exposed to the external surface of the skull. 



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 their 

 homologues in the human skull, the two bones are separate through- 

 out life, so that there is a permanent 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 part 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 wdth the nasal and premaxillary bones. Two processes 

 are thus formed, one medial, the other lateral to the nasal. The 

 medial process is associated with that of the opposite side to form 

 a triangular frontal spine, while the lateral or maxillary process 

 (processus maxillaris) projects forward between the nasal and 

 premaxillary bones, on the one hand, and the subcutaneous process 

 of the lacrimal, the orbital process of the maxilla, and the body of 

 the latter, on the other. 



The orbital portion of the frontal forms a considerable portion 

 of the orbital wall. Its anterior margin is in contact with the 

 lacrimal bone, its ventral margin with the slender sphenoorbital 

 process of the maxilla, the ethmoid process of the orbitosphenoid, 

 and the orbitosphenoid proper. Its internal surface is divided by 

 a vertical ridge into anterior and posterior portions, in relation 

 respectively to the anterior and middle cranial fossae. The anterior 

 cranial fossa is enclosed by the frontal bones, with the exception, 

 however, of a small portion of the floor which is formed by the 

 cribriform plate of the ethmoid. 



The Ethmoid Bone 

 The ethmoid bone (os ethmoidale), the chief representative of 

 the embryonic cartilaginous nasal capsule, is a delicate, greatly 



