70 



OS IIYOTDES. 



OS HYOIDES. 



The os hy aides forms the second arch developed from the cranium, 

 and gives support to the tongue, and attachment to numerous mus- 

 cles in the neck. It is named from its resemblance to the Greek let- 

 ter u, and consists of a central portion, or 

 Fig. 26 * 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 anterior surface, where it 

 gives attachment to muscles ; concave and 



smooth on the posterior surface, by which it lies in contact with the 

 epiglottis. The greater cornua are flattened from above downwards, 

 and terminated posteriorly in a tubercle; and the lesser cornua, 

 conical in form, give attachment to the stylo-hyoid ligaments. 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. 



Developement. By Jive centres, one for the body, and one for 

 each cornu. 



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. 



THORAX AND UPPER EXTREMITY. 



The bones of the thorax are the sternum and ribs; and, of the 

 upper extremity, the clavicle, scapula, humerus, ulna, and radius, 

 bones of the carpus, metacarpus, and phalanges. 



STERNUM. The sternum (fig. 27) is situated in the middle line of 

 the front of the chest ; it is flat, or slightly concave in front, and 

 convex behind ; broad and thick above, and flattened and pointed 

 below. It consists of three pieces ; superior, middle, and inferior. 



The superior (1) is nearly quadrilateral ; broad and thick above, 

 and somewhat narrowed at its junction with the middle piece. At 

 each superior angle is a deep articular depression for the clavicle, 

 and on either side two notches, for the articulation of the cartilage 

 of the first rib, and one half of the second. 



The middle piece (2), considerably longer than the superior, is 



* The os hyoides seen from before. 1. The anterior convex side of 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 was 

 drawn. 



