Skull and Hyoid 



273 



the bone, in that it does not define any muscular area, but (Fig. 217) is covered by one 

 of these on each side : the ridge connects the regions of the lesser cornua and probably 

 marks the basal part of the second bar. It is sometimes partly separated from the 

 rest of the body. The median vertical ridge, however, may be looked on as a secondary 

 line where it lies below the horizontal ridge ; but the part above this ridge, though 

 possibly in part secondary, may have some atavistic value, for it sometimes shows a 

 short process that has been homologised with the " entoglossal process " found in 

 some lower animals. 



The greater cornu presents a prominent angle, particularly in male bones, at its 

 outer and front part : this is made by the attachment of the expansions of Digastric 

 and Stylo-hyoid. 



The lower and back margin has the thyro-hyoid membrane attached to its whole 

 length. The upper surface has the Middle Constrictor arising from its length, and 



FIG. 216. i, sagittal section through the region of the mouth in an embryo of the fifth week, 

 showing the track of the median thyroid downgrowth ventral to the hyoid rudiment. 

 M. mandibular arch. 2, from a model exhibiting the thyroid stem in position as normally 

 seen, partly wrapped round the hyoid, but properly on the ventral aspect. Early in third 

 month. 



the fibres of the muscle cover the extremity of the cornu as they turn down into the wall 

 of the pharynx : all the origin of the Constrictor is hidden by the Hyo-glossus, which also 

 arises from the length of the cornu and extends on to the body for a little distance, 

 lying outside the lesser cornu. The lesser cornu and the stylo-hyoid ligament are in 

 the plane of the Middle Constrictor, which arises from them : the Digastric and Stylo- 

 hyoid are superficial to the Hyo-glossus and therefore cannot be attached to the small 

 cornu. Any fibres arising from the small cornu must be deep to Hyo-glossus, and thus, 

 beside the Constrictor, we find the Chondro-glossus arising from the inner aspect and 

 passing up to the deep aspect of the Hyo-glossus. 



The muscular areas on the bone are shown in Fig. 217 and a description given of the 

 limits of the areas : observe especially that Hyo-glossus reaches the body, Thyro- 

 hyoid does the same, Genio-glossus has a very small insertion, and Genio-hyoid has 

 the largest insertion on the body. 



The bone lies in the level of the second to third cervical vertebrae, and about 

 on a level with the lower margin of the lower jaw when the head is held in the natural 

 position. The body can be felt with difficulty from the surface owing to the covering 



F.A. 



18 



