96 PRACTICAL ANATOMY. 



ligaments. If a vertical section through the median line 

 of the sacrum is made, the sacral canal, continuous with 

 the spinal canal, is laid open. It is large above, but be- 

 comes rapidly contracted as it descends toward the apex. 

 The canal is flattened antero-posterioiiy ; opening into it 

 are the four pairs of anterior and posterior sacral fora- 

 mina for the transmission of the sacral nerves. 



The sacrum is dveloped by thirty-five centres, which 

 appear at the end of the second month of foetal life, and 

 in the upper segments of the bone. Each segment ossi- 

 fies separately ; they become joined together by distinct 

 ossific centres, the lowest two first, and this process ex- 

 tends upward until the five segments form the one bone, 

 which is not completed before the twenty-fifth year. 



The coccyx is a rudimentary bone and consists of four 

 segments ; its base articulates with the apex of the sacrum. 



BONES OF TRUNK. 

 THE HYOID BONE. 



The hyoid bone is situated in front of the bodies of 

 the third and fourth cervical vertebrae ; it gives attach- 

 ment to the muscles of the tongue, and is not articulated 

 with the skeleton. The bone forms a half-circle, the 

 convex portion presenting anteriorly. It consists of a 

 body, two greater and two lesser cornua. The body is 

 the central thick portion of the bone ; it is flattened, and 

 presents an anterior and posterior surface, superior and 

 inferior borders. The anterior surface, convex from 

 side to side, presents a median vertical ridge crossed by a 

 horizontal line dividing the surface into four shallow 

 fossae, which afford attachment to the muscles of the 

 tongue. The posterior surface is smooth and rounded, 

 and gives attachment to the thyro-hyoid membrane. 

 The superior border is bluntly rounded; the inferior 



