468 CHORDATA. 
the dentary and forms a fresh articulation, so that the 
quadrate is no longer necessary for this function, and passes 
backwards to form the ¢ympantc 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 maxille 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-palatine 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 VERTEBR#.—The cervical vertebre are usually 
seven in numbert and are distinguished from all other 
vertebree 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. 
+ Exceptions are found in the Zdendata and Sivenia. Bradypus has eight or 
nine, 7% dua eight, Cholapus and Manatius six. 
