468 CHORD AT A, 



the dentary and forms' a fresh articulation, so that the 

 quadrate is no longer necessary for this function, and passes 

 backwards to form the tympanic bone which surrounds the 

 outer part of the ear.^ 



The lower jaw also appears to consist of a single bone 

 on each side. 



In this connection we may note that the squamosal 

 articulation has the condyle on the movable part, whereas 

 the quadrate articulation of Sauropsida has the condyle 

 on the quadrate or immovable part. The first has a 

 mechanical advantage which may partially account for the 

 substitution. 



Other special points we may note in the skull of the 

 mammal are these: — The skull is suspended to the first 

 vertebra by two condyles borne on the two exoccipitals. 

 The maxillae and palatines meet their fellows across the roof 

 of the mouth to form a bony palate, so that the nasal 

 cavity only communicates with the buccal cavity by small 

 naso-palati?ie foramina in front and by the internal nares 

 behind. The maxilla and squamosal are connected across 

 under the orbit by the malar or jugal, forming a bridge of 

 bone called the zygomatic arch (or suborbital bar). Ridges 

 for the insertion of muscles may be formed, such as the 

 sagittal crest along the median dorsal line and the occipital 

 crest at right angles to it in the occipital region. These are 

 best developed when a heavy "bite" is required. The 

 tympanic bone very commonly expands into a swollen bulla 

 tympani below the ear, enclosing the tympanic cavity or 

 middle ear. 



The Vertebrae. — The cervical vertebrae are usually 

 seven in numbert and are distinguished from all other 

 vertebrae by having a pair of lateral foramina as well as the 

 large central one. These are known as the vertebrarterial 

 canals, because the vertebral artery runs through them. 

 They are formed by the cervical rib, with its head forked 

 into capitulum and tuberculum, becoming fused on to the 



* This is one of several views as to the fate of the quadrate in mammals. Many 

 hold that it forms the incus. ^ 



t Exceptions are found in the Edendata and Sirenia. Bradypns has eight or 

 nine, Tamandua eight, Cholocpus and Manatus six. 



