190 



THE SKELETON. 



Middle Constrictor of Pharynx 

 BuO'Glottiu 



Body 



Genio-Hyoid Sterno-IIyoid 



Thyro- Hyoid 

 Stijlo-Hijoid 



Omo-Hyoid 

 Mylo-Hyoid 



into two parts by a vertical ridge, which descends along the median line, and 

 crossed at right angles by a horizontal ridge, so that this surface is divided 



into four muscular depressions. 



Fig. 135. Hyoid Bone. Anterior Surface (rnhir-ied). At the point of meeting of these 



two lines is a prominent eleva- 

 tion, the tubercle. The portion 

 above the horizontal ridge is 

 directed upwards, and is some- 

 times described as the superior 

 border. The anterior surface 

 gives attachment to the Genio- 

 hyoid in the greater part of its 

 extent; above, to the Genio-hyo- 

 glossus; below, to the Mylo-hy- 

 oid, 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, 

 arid 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 back- 

 wards, 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 liga- 

 ments; 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 fcetal 

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

 commences 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 THORAX. 



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

 protect the principal organs of respiration and circulation. It is the second in 

 size 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, arid the bodies of 

 the dorsal vertebras behind. 



THE STEBNTJM. 



The Sternum (Figs. 136, 137") is a flat narrow bone, situated in the median 

 line of the front of the chest, and consisting, in the adult, of three portions. It 



