274 



Anatomy of Skeleton 



of muscle fibres, and the same is true of the greater cornu ; the small cornu is not 

 palpable. The cornua can be felt from within the pharynx (Fig. 219), but the body is 

 rather hidden from this aspect by the epiglottis : the great cornu lies under the pharyngo- 

 epiglottic fold and the mucous membrane in the upper part of the pyriform fossa, the 

 floor of which rests (below the hyoid) on the thyro-hyoid membrane and upper part 

 of the thyroid ala. 



Development. 



The mesenchymal structure shown in Fig. 218 is chondrified from centres that 

 appear in it in the fifth or sixth week. A single median cartilaginous centre is found 



. 



position of 



extremity aj- 

 re&Cornu. 



pyrt'for 



Supfriorcorrtu 

 of thyroi 



FIG. 219. Diagram of section through the pharynx in a sagittal direction to 

 show the position of the great cornu and upper cornu of thyroid (inter- 

 rupted lines) to the folds and fossae in that region. The cavity is shown 

 opened out. Fibres of Palatopharyngeus run down on the inner side of the 

 extremity of the greater cornu, under the mucous membrane. 



early in the back of the rudiment of the body. Other chondral centres appear (as seen 

 in Fig. 218) in the bars of the second and third arches. The whole structure is chondri- 

 fied from these centres during the third and fourth months, and at birth (Fig. 217) bony 

 centres have just appeared near the ventral ends of the great cornua. The body is 

 ossified from a single (? paired) centre showing shortly afterwards, but the small cornua 

 apparently do not commence to ossify until just before puberty. The extremity of 

 each great cornu remains cartilaginous until nearly middle life : epiphyses have been 

 described for these ends. 



The body and great cornua may fuse after middle life ; the small cornu may join 

 the greater, or in rare cases may fuse with the body. 



