OS HYOIDES. 99 



ciduous teeth. "These cords and foramina are not obliterated in the 

 child," says Mr. Goodsir, "either because the cords are to become 

 useful as < gubernacula* and the canals as 'itinera dentium? or, mudi 

 more probably, in virtue of a law, which appears to be a general one in the 

 development of animal bodies, viz. that parts, or organs, which have once 

 acted an important part, however atrophied they may afterwards become, yet 

 never altogether disappear, so long as they do not interfere with other parts 

 or functions " 



Succession. The periods of appearance of the teeth are extremely irre- 

 gular ; it is necessary, therefore, to have recourse to an average, which, 

 for the temporary teeth, may be stated as follows, the teeth of the lower 

 jaw preceding those of the upper by a short interval : 

 7th month, two middle incisors. 18th month, canine. 

 9th month, two lateral incisors. 24th month, two last molares. 



12th month, first molares. 



The periods for the permanent teeth are, 



6J year, first molares. 10th year, second bicuspides. 



7th year, two middle incisors. llth to 12th year, canine. 



8th year, two lateral incisors. 12th to 13th year, second molares. 



9th year, first bicuspides. 17th to 21st year, last molares. 



o s HYOIDES. 



The 05 hyoides forms the second arch developed from the cranium, and 

 gives support to the tongue, and attachment to numerous muscles in the 

 neck. It is named from its resemblance to the Greek letter u, and consists 

 of a central portion or body, of two larger cornua, which project backwards 

 from the body ; and two lesser cornua, which ascend from the angles of 

 union between the body and the greater cornua. 



The body is somewhat quadrilateral, rough 

 and convex on its antero-superior surface, by 

 which it gives attachment to muscles ; concave 

 and smooth on the postero-inferior surface, by 

 which it lies in contact with the thyro-hyoidean 

 membrane. The greater cornua are flattened 

 from above downwards, and terminated posteri- 

 orly by a tubercle ; and the lesser cornua, conical 

 in form, give attachment to the stylo-hyoid liga- 

 ments. In early age and in the adult, the cornua are connected with the 

 body by cartilaginous surfaces and ligamentous fibres ; but in old age they 

 become united by bone. 



Development. By five centres, one for the body, and one for each 

 cornu. Ossification commences in the greater cornua during the last 

 month of foetal life, and in the lesser cornua and body soon after birth. 



Attachment of Muscles. To eleven pairs ; sterno-hyoid, thyro-hyoid, 

 omo-hyoid, pulley of the digastricus, stylo-hyoid, mylo-hyoid, genio 

 hyoid, genio-hyo-glossus, hyo-glossus, lingualis, and middle constrictor 

 of the pharynx. It also gives attachment to the stylo-hyoid, thyro-hyoid, 

 and hyo-epiglottic ligaments, and to the thyro-hyoidean membrane. 



* The os hyoides seen from before. 1. The antero-superior, or convex side oi the 

 body. 2. The great cornu of the left side. 3. The lesser cornu of the same side. The 

 cornua were ossified to the body of the bone in the specimen from which this figure wa* 

 drawn. 



