112 OSTEOLOGY. 



prominent elevation, the tubercle. The portion above the horizontal ridge is 

 directed upwards, and is sometimes described as the superior border. The anterior 

 surface gives attachment to the Genio-hyoid in the greater part of its extent ; 

 abeve, to the Genio-hyo-glossus ; below, to the Mylo-hyoid, Stylo-hyoid, and 

 aponeurosis of the Digastric ; and between these to part of the Hyo-glossus. The 

 posterior surface is smooth, concave, directed backwards and downwards, and 

 separated from the epiglottis by the thyro-hyoid membrane, and by a quantity 

 of loose areolar tissue. The superior border is rounded, and gives attachment to 

 the thyro-hyoid membrane, and part of the Genio-hyo-glossi muscles. The 

 inferior border gives attachment, in front, to the Sterno-hyoid ; behind, to part 

 of the Thyro-hyoid, and to the Omo-hyoid at its junction with the great cornu. 

 The lateral surfaces are small oval, convex facets, covered with cartilage for 

 articulation with the greater cornua. 



The Greater Cornua project backwards from the lateral surfaces of the body ; 

 they are flattened from above downwards, diminish in size from before Dackwards, 

 and terminate posteriorly in a tubercle for the attachment of the thyro-hyoid 

 ligament. Their outer surface gives attachment to the Hyo-glossus ; their upper 

 border, to the Middle constrictor of the pharynx ; their lower border, to part of 

 the Thyro-hyoid muscle. 



The Lesser Cornua are two small conical-shaped eminences, attached by their 

 bases to the angles of junction between the body and greater cornua, and giving 

 attachment by their apices to the stylo-hyoid ligaments. In youth, the cornua are 

 connected to the body by cartilaginous surfaces, and held together by ligaments : 

 in middle life, the body and greater cornua usually become joined ; and in old 

 age, all the segments are united together, forming a single bone. 



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

 Ossification commences in the body and greater cornua towards the end of foetal 

 life, those of the cornua first appearing. Ossification of the lesser cornua com- 

 mences some months after birth. 



Attachment of Muscles. Sterno-hyoid, Thyro-hyoid, Omo-hyoid, aponeurosis 

 of the Digastricus, Stylo-hyoid, Mylo-hyoid, Genio-hyoid, Genio-hyo-glossus, 

 Hyo-glossus, Middle constrictor of the pharynx, and occasionally a few fibres of 

 the Lingualis. It also gives attachment to the thyro-hyoidean membrane, and 

 the stylo-hyoid, thyro-hyoid, and hyo-epiglottic ligaments. 



THE THOEAX. 



The Thorax or Chest is an osseo-cartilaginous cage, intended to contain and 

 protect the principal organs of respiration and circulation. It is the largest of 

 the three cavities connected with the spine, and is formed by the sternum and 

 costal cartilages in front, the twelve ribs on each side, and the bodies of the dorsal 

 vertebras behind. 



THE STERNUM. 



The Sternum (figs. 69 and 70) is a flat narrow bone, situated in the median line 

 of the front of the chest, and consisting, in the adult, of three portions. Its form 

 resembles an ancient sword : the upper piece, representing the handle, is termed 

 the manubrium; the middle and largest piece, which represents the chief part of the 

 blade, is termed the gladiolus; and the inferior piece, like the point of the sword, 

 is termed the ensiform or xiphoid appendix. In its natural position, its direction 

 is oblique from above, downwards, and forwards. It is flattened in front, concave 

 behind, broad above, becoming narrowed at the point where the first and second 

 pieces are connected ; after which it again widens a little, and is pointed at its 

 extremity. Its average length in the adult is six inches, being rather longer in 

 the male than in the female. 



The First Piece of the sternum, the Manubrium, is of a somewhat triangular 

 form, broad and thick above, narrow below at its junction with the middle piece. 

 Its anterior surface, convex from side to side, concave from above downwards, is 

 smooth, and affords attachment on each side to the Pectoralis major and sternal 



