=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 thoracic vertebræ 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 immovable joints, most of which disappear with age.