THE SKELETON 



21 



The occipital bone (Lat. ob, against ; and caput, head) forms tne back 

 and a part of the base of the skull. Its lower portion is perforated by a 

 large circular opening, about an inch and a half in diameter, called the 

 foramen magnum (Lat. foramen, a hole ; magnus, great), by which the 

 brain cavity is made to communicate with the spinal canal. On each side of 

 the front portion of this cavity is situated a rounded projecting mass of bone 

 called the condyle (Gr. a knuckle). These two condyles fit into two corre- 

 sponding depressions in the first cervical vertebra, thus forming a pair of 

 joints which permit of a rocking or nodding motion of the head. 



The upper edge of the occipital bone is united with the parietal bones 

 (Lat. paries, a wall), which form the side walls and the greater portion of the 

 roof of the cranium. 



The frontal bone (Lat. frons, the forehead) forms the front of the cranium. 

 It is united with the two parietals behind, and extends over the forehead, 

 forming the roofs of the sockets or orbits of the eyes. The frontal bone in a 



Fig. 21. - The Sphenoid Bone, viewed from before. 



young child is divided into two parts by a space which is continuous with the 

 line of division between the parietal bones. 



The base of the skull, in front of the occipital bone, is formed by the 

 tphenoid bone (Gr. sfhen, a wedge ; and eidos, form). It much resembles a 

 bat in shape. It is wedged in between the other bones of the base of the 

 skull, and extends forward to meet the frontal bone in the orbits. 



The ethmoid bone (Gr. ethmos, a sieve ; and eidos, form) fills the space 

 between the sockets, and is so called because it is perforated by a large 

 number of small openings, through which the branches of the nerve of smell 

 pass from the brain to the nose. This bone is very irregular in form, and 

 extends into the cavities of the nose. 



The temporal bones (Lat. tempics, time) are so called because they are 

 situated in those parts of the head (the temples) where the effects of time are 

 so often first shown by the appearance of white hairs. They are attached to 

 the sphenoid bone in front, the parietal bones above, and the occipital bone 

 behind. They also send out processes which unite with the cheek-bones, form- 

 ing bony arches. 



