THE SKELETON 23 



that the lower jaw has not only a vertical movement, but is capable also of 

 motion sideways and backwards and forwards. 



It will be observed that several bones enter into the formation of the orbits 

 or sockets of the eyes. The upper part of each orbit is formed by the frontal 

 and sphenoid bones ; the inner portion by the lachrymal plate ; the lower 

 portion by the superior maxillary ; and the outer portion by the malar or 

 cheek-bone. These orbits (Lat. orbita, a track in which a body turns) con- 

 tain the eyeballs, the muscles which turn the eyeball, and a certain amount of 

 fatty matter which acts as a padding to protect the delicate organ of vision. 



We must now notice how the head turns on the vertebral 

 column. It has already been seen that the skull rocks or nods 



Fig. 23. The Atlas, or First Cervical 

 Vertebra ; viewed from above. 



i, the anterior arch ; 2, the posterior arch ; 

 3, spinal cavity ; 4, lateral processes ; 5 marks 

 the position of the odontoid peg of the axis ; 

 6, concave surfaces which articulate with the 

 occipital bone. 1'he dotted line marks the 

 position of the ligament which secures the 

 peg. The body is absent, but is represented 

 by the odontoid peg of the axis. 



Fig. 24. The Axis, or Second Cervical Vertebra. 



A, viewed from above and behind ; B. viewed from the right side ; i, odontoid 

 process ; 2, body ; 3, spinal cavity ; 4, lateral processes ; 5, spinal process. 



on the vertebral axis by means of a pair of joints, formed by 

 projections (condyles) of the occipital bone, and the corresponding 

 depressions or sockets in the first cervical vertebra, the atlas. But, 

 were these the only joints permitting a movement of the head, 

 that motion would be restricted to nodding. We know, however, 

 that the head turns freely to right and left. This is explained as 

 follows. The second cervical vertebra, the axis, sends upward a 

 tooth-like projection called the odontoid process (Gr. odous, tooth ; 

 and eidcs, form). This peg forms an axis on which the atlas turns, 

 and is kept in its position against the front inner surface of the 

 atlas by means of a powerful ligament Thus, when the head 



