THE HEAD. 



Fig. 25. 



inferior incisors, and behind these, in male animals only, there is an 

 additional alveolus for the tusk. The portion included on each side between 

 the last incisor and first molar, forms a more or Jess sharp ridge, which 

 constitutes the inferior interdental space or bars. 



Structure and development. Formed, like all the flat bones, by two 

 compact plates separated by spongy tissue, the inferior maxilla is developed 

 from two centres of ossification, which correspond to each branch, and which 

 coalesce some time after birth. 



II. Tie Hyoid Bone. 



The hyoid bone constitutes a small and special bony apparatus which 

 serves to support the tongue, as well as the larynx and pharynx; its 

 description is placed immediately after that of the bones of the head 

 because of its connection with that region, it being situated between the two 

 branches of the supermaxillary bone, and suspended to the base of the cra- 

 nium in an oblique direction from above to below, and from before to behind. 



This apparatus is composed of seven distinct pieces, arranged in three 

 series : a middle, constituted by a single bone, and named the body ; two 

 lateral, forming two quasi-parallel branches, to the extremities of which the 

 body is articulated. 



Body. The body of the hyoid resembles a fork with two prongs. It 

 presents : 1, A middle part flattened above and below, and consequently pro- 

 vided with a superior and an 

 inferior face : 2, A single and 

 long prolongation flattened 

 on both sides, which is de- 

 tached from the middle part, 

 and directed forward and 

 downward to plunge into the 

 muscular tissue of the 

 tongue : this is the anterior 

 appendix of the hyoideal body ; 

 3, Two lateral cornua, thyroid 

 cornua, or great cornua, pro- 

 jecting backwards and up- 

 wards, articulating by their 

 extremities with the thyroid 

 cartilage of the larynx, and offering, at their point of union with the 

 middle part, two convex diarthrodial facets looking upwards, and corre- 

 sponding with the styloid cornua. The body of the hyoid bone is developed 

 by three centres of ossification, a middle, and two lateral for the cornua. 



Branches. The three pieces composing these are articulated end to 

 end, by means of a cartilaginous substance that joins them together ; they 

 are of very unequal dimensions. The first, which is in relation with the 

 body, is of medium size, and is named the styloid cornu, small cornu, or small 

 branch. The second, termed the styloid nucleus, is the smallest. The third, 

 the largest, constitutes the styloid process, or bone, or great branch. 



1. The styloid cornu is a small cylindrical piece bearing a concave 

 diarthrodial surface on its inferior extremity to unite it to the body ; it is 

 very spongy, and is developed from two ossifying centres, one of which, the 

 epiphysary, is for the inferior extremity. 



2. The styloid nucleus, which is often absent, is imbedded in the uniting 

 cartilaginous substance. 



7 



HYOID BONE. 



1, 1, Superior extremity of the styloid bones , 2, Angle 

 described by their posterior border; 3, 3, Styloid 

 bones ; 4, 4, Extremities of the thyroid cornua ; 5, 

 Articular surface of the body corresponding with the 

 small branches ; 6, Anterior appendix of the body ; 

 7, 7, Small branches, or styloid cornua. 



