Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/44

30 creases in size, and air enters the lungs, when the ribs are pulled anteriorly in breathing.

Exercise VII. Sketch one of the longer ribs.

The pectoral girdle includes the scapula (shoulder blade), and the clavicle (collar bone). The former is a flattened triangular bone with its apex pointing cranioventrad and its broad inner surface facing the vertebral column. A cartilaginous disc connects one end of the clavicle with the acromion process of the scapula. A similar disc unites the other end with the anterior end of the sternum. The clavicle is the only bony connection between the scapula and axial skeleton.

The concave thickened superior margin of the scapula is directed cephalad. The convex vertebral margin is dorsal. The third margin, the axillary faces ventrocaudally, and is nearly straight. The anterior, posterior, and ventral corners of the bone are designated the medial, inferior, and lateral angles respectively. At the lateral angle is the pear-shaped glenoid cavity for articulation with the humerus bone of the upper arm. The short coracoid process (homologous to the bone of the same name in the lower animals) projects medially from the cranial end of the glenoid cavity. The scapular spine is a thin, prominent plate (forming nearly a right angle with the scapula) which divides the lateral surface of the scapula into the supraspinous fossa (anterior) and the concave infraspinous fossa (posterior). The spine tapers dorsally to a point at the vertebral border; it ends ventrally in a free projection, the acromion process or acromion. The inconspicuous projection on the caudal surface of the latter is probably the metacromion process. There are two longitudinal ridges on the inner surface of the scapula.