402 THE VERTEBRATE SKELETON. 



perforate the palatine and transmit branches of the trigeminal 

 nerve and certain blood-vessels. 



f. The posterior narial opening is bounded chiefly by 

 the palatines. 



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 ver- 

 tebral 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 





