THE ORBITAL CAVITIES. 71 



sinuses. At each side of the body of the sphenoid, ex- 

 tending from the anterior lacerated foramen backward in 

 a sinuous manner, is the groove for the cavernous sinus. 

 In addition to these grooves, upon the inner surface of 

 the cranium we have canals ramifying freely in the diploic 

 structure of the cranial bones, which lodge veins or blood- 

 sinuses ; these blood-channels can be exposed by rasping 

 away the outer table of the cranial bones. 



The base of the skull is exceedingly interesting. A 

 line drawn through the apices of the mastoid processes 

 will bisect the foramen magnum and pass just behind 

 the occipital condyles. In the lowest races, as the Austra- 

 loid, this line passes through the condyles near their 

 anterior border, owing to the recession of the foramen 

 magnum in the lower races. A line drawn transversely 

 through the auditory meatuses will pass through the 

 jugular foramina, the anterior part of the occipital con- 

 dyles, and touch the anterior margin of the foramen 

 magnum. A line drawn just in front of the glenoid 

 fossae will pass through the foramen ovale and the line 

 of junction between the occipital and sphenoid bones. 

 The plane of the base of the cranium anteriorly is on a 

 line with the upper border of the zygomatic processes. 



Between the superior and inferior surfaces of the 

 base anteriorly is the facial portion of the skull, present- 

 ing the openings of the orbital and nasal cavities. 



The orbital are two pyramidal cavities below the 

 frontal bone. Their axes diverge and are directed out- 

 ward, forming an angle of 40 degrees. The apex is 

 posterior at the optic foramen. The base is the large, 

 irregularly circular opening which forms the circumfer- 

 ence of the orbit. Seven bones enter into the formation 

 of each orbital cavity. They are the frontal, sphenoid, 

 malar, superior maxillary, ethmoid, lachrymal, and 



