28 OSTEOLOGY 



asternal ribs overlap and are attached to each other to form the costal arch (Arcus 

 costalis). The cartilages of floating ril.)s are not attached to those adjacent. 



THE STERNUM 



The sternum (or breast-bone) is a median segmental bone which completes the 

 skeleton of the thorax ventrally, and articulates with the cartilages of the sternal 

 ribs laterally. It consists of six to eight bony segments (Sternebrse) connected by 

 intervening cartilage in the young subject. Its form varies with that of the thorax 

 in general and with the development of the clavicles in animals in which these bones 

 are present. Its anterior extremity, the manubrium stemi or presternum, is 

 specially affected by the latter factor, being broad and strong when the clavicles 

 are well developed and articulate with it (as in man), relatively small and laterally 

 compressed when they are absent (as in the horse) or rudimentary (as in the dog) . 

 The cartilages of the first pair of ribs articulate %vith it. The body or mesostemum 

 (Corpus sterni) presents laterally, at the junction of the segments, concave facets 

 (Incisurse costales) for articulation ivith the cartilages of the sternal ribs. The 

 posterior extremity or metastemum presents the xiphoid cartilage (Processus 

 xiphoideus) ; this is thin and wide, as in the horse and ox, or narrow and short, as 

 in the dog. 



Development. — The cartilaginous sternum is formed by the fusion mediallj' 

 of two lateral bars which unite the ventral ends of the first eight or nine costal 

 cartilages, and is primitively unsegmented. The manubrium ossifies from a single 

 center, but the centers for the other segments appear to be primitively paired. 

 The sternum never becomes completely ossified; details in regard to persisting 

 cartilage ■nail be given in the special descriptions. The layer of compact tissue is 

 for the greater part very thin and the spongy substance is open-meshed and very 

 vascular. 



THE THORAX 



The skeleton of the thorax comprises the thoracic vertebrse dorsally, the ribs 

 and costal cartilages laterally, and the sternum ventrally. The thoracic cavity 

 (Cavum thoracis) resembles in shape an irregular truncated cone; it is compressed 

 laterally, especially in front, and the dorsal wall or roof is much longer than the 

 ventral wall or floor. The anterior aperture or inlet (Apertura thoracis cranialis) is 

 bounded by the first thoracic vertebra dorsally, the first pair of ribs and costal 

 cartilages laterally, and the manubrium sterni ventrally. The posterior aperture 

 (Apertura thoracis caudalis) is bounded b}^ the last thoracic vertebra, the last 

 pair of ribs, the costal arches, and the anterior part of the xiphoid cartilage. 



It may be noted here that the diaphragm (which forms the partition between the thoracic 

 and abdominal cavities) does not follow the costal arches in its posterior attachment, so that the 

 posterior ribs enter also into the formation of the abdominal wall. 



THE SKULL 



The term skull is usually understood to include all of the bones of the head. 

 The head consists of the cranium and the face, and it is therefore convenient to 

 divide the bones into cranial and facial groups. 



The cranial bones (Ossa cranii) inclose the brain with its membranes anrl 

 vessels and the essential organs of hearing. They concur with the facial bones in 

 forming the orbital and nasal cavities, in which the peripheral organs of sight and 

 of smell are situated. 



The facial bones (Ossa faciei) form the skeleton of the oral and nasal cavities 

 and also support the pharynx, the larynx, and the root of the tongue. 



