g. The alisphenoid canal (fig. 75, 21) is a short canal penetrating the base of the alisphenoid bone, and transmitting the external carotid artery. It lies between the foramen rotundum and the foramen ovale.
h. Between the auditory bulla and the foramen ovale are seen two openings. The more external of these is the opening of the Eustachian canal (fig. 75, 22), which communicates with the tympanic cavity. The more internal is the foramen lacerum medium (fig. 75, 9), through which the internal carotid enters the cranial cavity.
i. The external auditory aperture (fig. 75, 7) is a large opening with rough edges at the outer side of the tympanic bulla.
j. Between it and the glenoid surface of the squamosal is the postglenoid foramen (fig. 75, 10) through which a vein passes out.
k. Lastly, there is the great foramen magnum (fig. 75, 2), between the occipital condyles. Through it the brain and spinal cord communicate.
C. The Ribs and Sternum.
These, together with the thoracic vertebrae, form the skeletal framework of the thorax. Each rib is a curved rod, which at its dorsal end is movably articulated to the vertebra, and at its ventral end is either connected with the sternum, or ends freely. In the dog there are thirteen pairs of ribs, nine pairs of which are directly connected with the sternum, while the remaining four end freely and are known as floating ribs. Each rib is obviously divided into two parts, a dorsal or vertebral part, and a ventral or sternal part. The vertebral portion, which forms about two-thirds of the whole rib, is a flattened, regularly curved rod, completely ossified. Its dorsal end is rounded, forming the head or capitulum, which articulates with a concave surface furnished partly by the corresponding vertebra and partly by the vertebra next in front. The last three or four however articulate with one vertebra only. A short way behind the capitulum on the dorsal side of the rib is a rounded outgrowth, the tubercle or tuberculum, by means of which the rib articulates with the transverse process. The portion of the rib between the head and the tubercle is known as the neck. The sternal portion of the rib (fig. 76) is a short bar of calcified or imperfectly ossified cartilage, about one-third of the length of the corresponding bony portion. The anterior sternal ribs are somewhat more cartilaginous than the posterior ones. The vertebral portions increase in length from the first which is very stout, and has the capitulum and tuberculum very distinct, to about the eighth or ninth; afterwards they gradually diminish in size. The first nine to eleven have the capitula and tubercula separate, afterwards they gradually merge together.
Fig. 76. Sternum and sternal ribs of a Dog (Canis familiaris) × ½.