joint. The
_Radius_, so called from its resemblance to a spoke, is on the outer
side of the fore-arm, and articulates with the bones of the wrist,
forming a joint. The ulna and radius also articulate with each other at
their extremities. The _Carpus_, or wrist, consists of eight bones,
arranged in two rows. The _Metacarpus_, or palm of the hand, is composed
of five bones situated between the carpus and fingers. The _Phalanges_,
fourteen in number, are the bones of the fingers and thumb, the fingers
each having three and the thumb two.

THE BONES OF THE LOWER EXTREMITIES, sixty in number, are classed as
follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and
Phalanges. The _Femur_, or thigh-bone, is the longest bone in the body.
It has a large round head, which is received into the acetabulum, thus
affording a good illustration of a ball and socket joint. The _Patella,_
or knee-pan, is the most complicated articulation of the body. It is of
a round form, connects with the tibia by means of a strong ligament, and
serves to protect the front of the joint, and to increase the leverage
of the muscles attached to it, by causing them to act at a greater
angle. The _Tibia_, or shin bone, is enlarged at each extremity and
articulates with the femur above and the astragalus, the upper bone of
the tarsus, below. The _Fibula_, the small bone of the leg, is situated
on the outer side of the tibia, and is firmly bound to it at each
extremity. The _Tarsus_, or instep, is composed of seven bones, and
corresponds to the carpus of the upper extremities. The _Metatarsus_,
the middle of the foot, bears a dose resemblance to the metacarpus, and
consists of five bones situated between the tarsus and the phalanges.
The tarsal and the metatarsal bones are so united as to give an arched
appearance to the foot, thus imparting elasticity. The _Phalanges_, the
toes, consist of fourteen bones, arranged in a manner similar to that of
the fingers.

We are not less interested in tracing the formation of bone through its
several stages, than in considering other parts of the human system. The
formation of the Haversian canals for the passage of blood-vessels to
nourish the bones, the earlier construction of bony tissue by a
metamorphosis of cartilaginous substance, and also the commencement of
ossification at distinct points, called _centers of ossification_, are
all important subjects, requiring the student's careful attention. The
bones