28 OSTEOLOGY 
asternal ribs overlap and are attached to each other to form the costal arch (Arcus 
costalis). The cartilages of floating ribs 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 (Sternebre) 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 sterni 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 with it. The body or mesosternum 
(Corpus sterni) presents laterally, at the junction of the segments, concave facets 
(Incisurze costales) for articulation with the cartilages of the sternal ribs. The 
posterior extremity or metasternum 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 medially 
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 will 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 thoracie vertebre 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 by the last thoracie 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 and 
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. 
