8 
MAMMALS. 
atlas; following which is the axis vertebra, remarkable for having the body or 
basal portion of the atlas vertebra fixed to it, and known as the odontoid process. 
This separation of the body of the atlas vertebra from its proper segment is 
constant throughout the greater part of the vertebrate subkingdom. The remaining 
five of the cervical, or neck-vertebrae, are distinguished from the dorsal, or vertebrae 
of the region of the chest, by the absence of ribs. The ribs of most of the dorsal 
vertebrae articulate in the middle line of the inferior aspect of the body with the 
breast-bone, or sternum, which is itself composed of several segments. The dorsal 
vertebrae are succeeded posteriorly by a smaller number, forming the region of the 
loins, which have no ribs, and are termed lumbars. Behind the latter there are 
several coalesced vertebrae forming the so-called sacrum, to which the haunch-bones 
articulate; and these are again succeeded by the tail, or caudal, vertebrae, of which 
the number varies according to the length of the tail itself. 
In the majority of Mammals the fore-limb is 
connected with the trunk simply by the blade-bone, 
or scapula, which lies on the back surface of the 
anterior ribs; and in front by the collar-bone, or 
clavicle, which connects the scapula with the ster¬ 
num. The bones of the fore-limb are, firstly, the 
arm-bone or humerus, which has condyles at its lower 
end; and, secondly, the two parallel bones of the 
fore-arm, of which the outermost (when the palm of 
the hand is turned forwards) is the radius, and the 
other the ulna. The radius is always present, but 
in many Hoofed Mammals only the upper end of the 
ulna remains, which is fused with the radius. The 
radius articulates below with the upper of the two 
transverse rows of small solid bones forming the 
wrist or carpus; beyond these we have in man and 
monkeys, as well as in certain other groups, five 
elongated bones, termed metacarpals, the four outer¬ 
most of which are succeeded by the three phalangeal 
bones of the fingers or digits. The thumb, or first 
digit, which lies on the same side as the radius, has, 
however, only two of these phalangeals. 
The hind-limb differs from the fore-limb in that 
the innominate, or haunch - bones, which together 
form the pelvis, are connected by an immovable bony 
union with the sacral region of the vertebral column. The thigh-bone or femur, 
corresponding to the humerus of the arm, articulates with a cavity in the innominate, 
termed the acetabulum. The leg has two parallel bones articulating with the 
lower end of the thigh-bone or femur; of which the larger, or tibia, occupying the 
inner side of the limb, corresponds to the radius of the fore-arm, while the smaller 
outer bone, or fibula, represents the ulna. The ankle, or tarsus, corresponds to the 
carpus in the fore-limb, and likewise consists of two transverse rows of small 
bones. Two bones of the uppermost row, viz. the calcaneum or heel-bone, and 
DIAGRAM OF THE BONES OF THE LEFT 
FORE-FOOT OF A FIVE-TOED MAM¬ 
MAL (Phenacodus). nat. size.) 
The bones marked s, l, c, t, t d , m. 
and u, form the wrist; those numbered 
I-V are the metacarpals, and the re¬ 
mainder the finger or toe - bones.— 
After Osborn. 
