SPINAL COLUMN. 33 



carrying the skull with it, rotates when the head is 

 turned from side to side. 



On the {interior (upper) surface of the atlas are a pair 

 of shallow hollows, Fas ; into these fit a pair of knobs, 

 found towards the back of the under surface of the skull 

 (Fig. 20), vvhich glide in the hollows during nodding 

 movements of the head. 



Uses of the mode of structure of the spinal column. 

 When the backbone is viewed from one side (Fig. 9) it is 

 seen to present four curvatures ; one in the neck, convex 

 ventrally, is followed by a curve in the opposite direction 

 in the dorsal region ; in the loins the curvature is again 

 convex ventrally, and opposite the sacrum and coccyx the 

 reverse is the case. These curves add greatly to the 

 springiness of the spine, and prevent the transmission of 

 sudden jars along it.* The compressible elastic pads placed 

 between the centra of the vertebrae promote the same end ; 

 the skull, containing the soft brain (which would be 

 readily injured by mechanical violence) and the spinal 

 cord, contained in the backbone itself, are thus protected 

 from jarring in running, jumping, &c. 



The compressible pads between the bodies of the verte- 

 bras allow of a certain range of movement between each 

 pair, so that the column as a whole may be bent to a con- 

 How is the skull articulated to the backbone? 

 How many curvatures are there in the backbone? What is their 

 direction? 



What results from the curvatures of the spinal column? What is 

 the object of the pads between the vertebrae? 



* Take a straight but tolerably flexible and elastic bar, as a lath, or, better still, a 

 thin steel rod. Hold it vertical, with one end resting on the floor, and give a smart 

 blow on the upper end ; the jar will be sudden and violent. Now bend the rod and 

 hit it again ; the jar will be much less, as the curved rod yields somewhat to the 

 blow on its top. 



