272 



Anatomy of Skeleton 



hyoid membrane attached to it, a prominent outer margin which affords insertion to 

 Stylo-hyoid and Digastric, a smoother inner edge that is in relation with the mucous 

 membrane of the pharynx, and an enlarged extremity which is fastened to the thyroid 

 cartilage by the thyro-hyoid ligament. 



The lesser cornu is fastened by fibrous tissue to the body and great cornu, and by 

 the stylo-hyoid ligament to the styloid process. It affords origin to some fibres of the 

 Middle Constrictor and the Chondro-glossus. 



The hyoid bone is an ossification of the ventral portions of the cartilaginous 

 bars of the second and third pharyngeal arches : the dorsal end of the second bar 

 is fused with the petro-mastoid and forms the styloid process, while the intermediate 

 part is the stylo-hyoid ligament ; the upper end of the third bar is only indicated 

 for a short period as a condensation of cells that ultimately disappears entirely. 

 The lesser cornua (cerato-hyals) are parts of the second bars, while each greater 

 cornu (thyro-hyal) represents the third bar of its side. The body is sometimes termed 

 the basihyal, but this name should more properly be applied to the transverse ridge 

 on the body, which probably alone represents the ventral junction of the two tars of 

 the second arches (Fig. 217). The more complete development of the second bar 



causes the small cornua to 

 appear longer at one stage of 

 their formation than the great 

 cornua (Fig. 218), but it must 

 of course be remembered that 

 the contrast is ma~'nly in appear- 

 ance only, because there is no 

 bone in the young specimen, 

 and in the adult the large second 

 bar is as fully represented, 

 although by more sharply 

 differentiated bony and liga- 

 mentous tissues. 



greyer eotnui. 



^ , . 

 ooiffro-tnftrior . 



Jvrf3.tj!.'0r body. 



FIG. 215. Female hyoid from the front and from behind. 



The relation of the thyroid (median) down-growth to the hyoid is of some interest 

 and importance ; it is frequently said to lie in or even behind the bone, but this is 

 probably not a true statement of the case. In the early period, when the anlage 

 of the hyoid is first recognisable, the thyro-glossal duct can be found passing altogether 

 ventral to the rudiment of the bone. This is shown in Fig. 216. At a later stage 

 the epithelial rod that runs down to the thyroid gland is still ventral to the hyoid 

 but is in close contact with it, and is in fact folded somewhat round its lower edge 

 (Fig. 216), so that a potential thyroid tube might remain behind the plane of the body 

 although it has not got there by growing down behind the hyoid. This is the condition 

 found in all normal embryos and young fcetuses, and the occurrence of thyroid cysts * 

 or fistulae are persistences of this earlier state. 



The bursa that lies behind the body of the bone is median in position, and for this 

 reason is probably not derived from the thyro-glossal duct : on each side of it the hollow 

 in the bone is filled by fibro-fatty tissue. The line of attachment of the thyro-hyoid 

 membrane can usually be made out above the hollow, and in the central part some 

 fibrous tissue connects the bone with the epiglottis. The front surface is practically 

 altogether muscular, and the transverse ridge is plainly not a secondary marking on 



* Such cysts are usually unilateral and so far at least as I have had opportunity of examining them 

 even when they seem to extend up behind the bone the proper relation may be made out with a little trouble 

 and the continuity of the bone established behind them. 



