124 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



/) Foramen lacerum. In front of the tympanic bulla on the ventral surface 

 and including the foramen ovale of other mammals; for the passage of part of 

 the fifth nerve and an artery. 



k} External carotid foramen. A small foramen in the middle of the medial 

 surface of the bulla; for the passage of the internal carotid artery, the other end 

 of the canal lying in the foramen lacerum. 



1} Jugular foramen. Just posterior to the preceding, in the depression 

 between occipital condyle and bulla; for the ninth, tenth, and eleventh cranial 

 nerves, and a vein. 



m) Hypoglossal canal. Including small apertures posterior to the preceding 

 for the passage of the twelfth (hypoglossal) nerve. 



n) Stylomastoid foramen. In front of the middle of the mastoid process, for 

 the passage of the seventh nerve. 



10. The lower jaw. The lower jaw or mandible consists of a single pair of 

 bones, the dentaries, fused in front by a symphysis. All other bones seen in the 

 lower jaw of the alligator have vanished (except the articular which is supposed 

 to be the malleus of the middle ear). The horizontal part of the mandible is 

 named the body, the vertical part, the ramus. (In the lower vertebrates each 

 half of the lower jaw is also named ramus.) The posterior end of the mandible 

 (cat) extends dorsally into a strong coronoid process, which in the natural position 

 projects into the temporal fossa. In the rabbit this is reduced to a slight projec- 

 tion which forms the lateral boundary of a deep groove. The articulating 

 surface of the mandible is borne on the condyloid process. The depressed areas 

 in the posterior part of the mandible are for the insertion of the muscles of 

 mastication. Near the anterior tip of the mandible on the outer surface is the 

 mental foramen (or two in the cat) through which the nerve of the lower jaw 

 exits. Near the caudal end of the inner surface is the mandibular foramen 

 through which the nerve enters and pursues a course in the interior of the man- 

 dible to the mental foramen. In the rabbit there is an additional foramen 

 just above the mandibular foramen, for the passage of a vein. 



Owing to the absence of the quadrate and of all of the bones of the lower 

 jaw except the dentary, the articulation of the lower jaw to the skull is between 

 the dentary and the squamosal. This feature distinguishes mammals from all 

 other vertebrates, for in the latter the articulation is between the articular and 

 the quadrate. The condition found in mammals is, however, approached by 

 those reptiles directly ancestral to mammals. 



11. The teeth. The teeth of mammals possess several characteristics: 

 they are set in sockets or alveoli in the jaws, a condition known as thecodont; 

 they are heterodont, that is, differentiated into several different kinds; they can 

 generally be replaced only once; and some of them are complicated in form. 

 The teeth of mammals furnish important taxonomic characters. Those of the 

 cat and rabbit are very different, the former having teeth characteristic of 



