THE THORAX — THE BONES OF THE THORACIC LIMB 27 



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 thoracic vertebrae dorsally, the ribs 

 and their 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 (Apertura thoracis cranialis) is 

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

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

 (Apertura thoracis caudalis) is bounded by 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 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 larynx and the root of the tongue. 



Most of the bones of the skull are flat bones, developed in membrane; those 

 of the cranial base may be classed as irregular, and are cartilage bones. Only 

 two form permanent movable joints with other parts of the skull. The mandible 

 or lower jaw-bone forms diarthrodial joints with the temporal bones, and the hyoid 

 bone is attached to the latter by bars of cartilage. The other bones form immov- 

 able joints, most of which disappear with age. 



In order to study the separate bones, skulls of young subjects are necessary, since later most 

 of the lines of demarcation become effaced. The relations of each bone to its surroundings should 

 be specially noted, since the final object is to understand the skull as a whole. In the descriptions 

 which follow the skull is considered with its long axis horizontal, and that of the horse will serve 

 as a type. 



THE BONES OF THE THORACIC LIMB 



The thoracic limb consists of four chief segments, viz., the shoulder girdle, 

 the arm, the forearm, and the forefoot or manus. 



The shoulder girdle (Cingulum extremitatis thoracicse), when fully developed, 

 consists of three bones — the scapula or shoulder-blade, the coracoid, and the 

 clavicle or collar-bone. In the domesticated mammals only the scapula, a large, 

 flat bone, is well developed, and the small coracoid element has fused with it, while 

 the clavicle is either absent or is a small rudiment embedded in the mastoido- 

 humeralis muscle. There is therefore no articulation of the shoulder with the 

 axial skeleton. 



The shoulder girdle is fully developed in birds and the lower mammals (monotremata). In 

 the higher mammals the coracoid is reduced to the coracoid process of the scapula, and the develop- 

 ment of the clavicle is in conformity with the function of the limb. Thus in typical quadrupeds, 



