Pn 4- o YI r\ Q 



THE SKULL AS A WHOLE. 159 



fat and a bursa, which separates this aspect from the thyreo-hyoid membrane. The 

 upper border, usually described with the anterior surface, is broad ; it is separated 

 from the anterior aspect by a transverse ridge, behind which are the impressions 

 for the attachment of the genio-hyoid muscles. Its posterior edge is thin and 

 sharp ; to this, above, are attached the genio-glossi, whilst behind and below the 

 thyreo-hyoid membrane is connected with it. The inferior border is well defined 

 and narrow ; it serves for the attachment of the omo-hyoid, sterno-hyoid, thyreo- 

 hyoid, and stylo-hyoid muscles. 



The greater cornua are connected on either side with the lateral parts of the 

 body. At first, union is effected by synchondroses, which, however, ultimately 

 ossify. These cornua curve backwards, as well as upwards, and terminate in more or 

 less rounded and expanded extremities. Compressed laterally, they serve for the 

 attachments laterally of the thyreo-hyoid and hyo-glossi muscles, and the middle 

 constrictor of the pharynx from below upwards, whilst medially they are con- 

 nected with the lateral expansions of the thyreo-hyoid membrane, the free edges of 

 which are somewhat thickened, and connect the extremities of the greater cornua 

 with the ends of the superior cornua of the thyreoid cartilage below. 



The lesser cornua, frequently cartilaginous in part, are about the size of grains 

 of wheat. They rest upon the upper surface of the bone at the junctions of the 

 greater cornua with the body. In youth they are separated from, but in advanced 

 life become ossified with, the rest of the bone, from which they are directed upwards, 

 backwards, and a little laterally. Their summits are connected with the stylo- 

 hyoid ligaments ; they also serve for the attachment of muscles. 



Connexions. The hyoid is slung from the styloid processes of the temporal bones by the 

 stylo-hyoid ligaments. Inferiorly it is connected with the thyreoid cartilage of the larynx by 

 the thyreo-hyoid ligaments and membrane. Posteriorly it is intimately associated with the 

 epiglottis. 



Ossification. In considering the development of the hyoid bone it is necessary to refer 

 to the arrangement and disposition of the cartilaginous bars of the second and third visceral 

 tarches. That of the second visceral arch, the hyoid bar or Reichert's cartilage, as it is 

 sometimes called is united above to the petrous part of the temporal, whilst ventrally it is 

 joined to its fellow of the opposite side by an independent median cartilage. Chondrifica- 

 tion of the third visceral arch only occurs towards its ventral extremity, forming what is 

 known as the thyreo-hyoid bar. This also unites with the median cartilage above mentioned. 

 In these cartilaginous processes ossific centres appear in certain definite situations. 

 Towards the end of foetal life a single centre (by some authorities regarded as primarily 

 double) appears in the median cartilage, and forms the body of the bone (basihyal). 

 About the same time ossification begins in the lower ends of the thyreo-hyoid bars, and 

 from these the greater cornua are developed (thyreo-hyals). During the first year the lower 

 ends of the hyoid bars begin to ossify and form the lesser cornua (cerato-hyals). The 

 cephalic ends of the same cartilages meanwhile ossify to form the styloid process 

 (stylohyal) on either side and one of the auditory ossicles called the stapes, whilst the 

 intervening portions of cartilage undergo resorption and become converted into the 

 .fibrous tissue of the stylo-hyoid ligaments, which in the adult connect the lesser cornua 

 'with the styloid processes of the temporal bone. The greater cornua fuse with the body 

 in middle life ; the lesser cornua only at a more advanced period. Variations in the 

 course of development lead to interesting anomalies of the hyoid apparatus. The lesser 

 cornua may be unduly long or the stylo-hyoid ligament may be bony ; in this case the 

 cartilage has not undergone resorption, but has passed on to the further stage of ossifica- 

 tion, thus forming an epihyal element comparable to that in the dog. The ossified 

 stylo-hyoid ligament, as felt through the pharyngeal wall, may be mistaken for a 

 foreign body. (Farmer, G. W. S., Brit. Med. Journ. 1900, vol. i. p. 1405.) 



THE SKULL AS A WHOLE. 



The skull as a whole may be studied as seen from the front (norma frontalis) 

 from the side (norma lateralis), from the back (norma occipitalis), from above 

 (norma verticalis), and from below (norma basalis). 



