THE HYOID BONE. 61 



The angle of the jaw is in the adult usually about 120 ; in infancy it is as great as 140 

 or more ; in strongly developed jaws it may be diminished to 110 or less ; and in old and 

 toothless jaws it is increased. These changes are connected with a variety of circumstances, 

 among which may be noticed, the development of the temporary and permanent teeth, the 

 absorption of the alveolar arch after the loss of the teeth in advanced age, the elongation of 

 the face and upper jaw towards adult life, and the varying state of development of the 

 masseter muscles at different periods (see also p. 78). 



THE HYOID BONE. 



The hyoid bone, or os linguce, is situated at the base of the tongue, and may 

 be felt between the chin and the thyroid cartilage. It is shaped like the letter IT, 

 and consists of a body and two pairs of cornua. It is suspended from the tips of 

 the styloid processes of the temporal bones by a pair of slender bands, the stylo- 

 hyoid ligaments, which in most animals form 

 distinct bones. 



The ~body, or central piece, is compressed 

 from before backwards, and lies in a plane 

 directed downwards and forwards. Its anterior 

 surface is convex, and marked by a transverse 

 ridge, with a slight median projection, on each 

 side of which are depressions for muscular 

 attachments. Its posterior surface is concave, 

 and looks towards the epiglottis. 



The great cornua project backwards from the Fig . 64 ._ THE HYOID BONE, FROM ABOVE 

 sides of the body. They are compressed from AND BEFORE. (Drawn by D. Gunn. ) 

 above down, are largest near their junction with 

 the body, and terminate behind in slightly expanded and rounded extremities. 



The small cornua, or cornicula, short and conical, project upwards and back- 

 wards from the places of junction of the body with the great cornua, and give 

 attachment at their extremities to the stylo-hyoid ligaments. They are commonly 

 in part, and not unfrequently entirely, cartilaginous. 



The great cornua are connected to the body by synchondrosis, and after middle life usually 

 by bony union ; the small cornua by a synovial articulation which is seldom anchylosed. 

 In some cases a synovial joint is formed also between the great cornu and body. 



TEE SKULL AS A WHOLE. 



THE SUTUBES. 



With the exception of the lower jaw, which is moveably articulated with the 

 temporal bone, the bones of the skull are closely fitted together by more or less 

 uneven edges or surfaces, there being generally interposed only a small quantity of 

 fibrous tissue, continuous with the periosteum ; and to these lines of articulation 

 the name suture is given. At the base of the cranium, however, in young subjects, 

 the basilar process of the occipital is connected to the sphenoid, and the jugular 

 process to the petrous, by a thin layer of cartilage ; the articulation is therefore 

 synchondrosis, and when adult age is reached it becomes converted into bony union. 



The sutures are best named from the bones between which they lie, as, occipito- 

 parietal, occipito-mastoid, fronto-ethmoidal, &c. Those around the parietal bones 

 are the longest and most regular, and to them special names have been applied ; 

 thus, above, between the two parietal bones, is the sagittal or interparietal suture ; 

 posteriorly is the deeply serrated lamMoid or occipito-parietal suture ; anteriorly is 

 the coronal or fronto-parietal suture, most markedly serrated in the middle part of 



