dorsal bones of the spinal
column behind, bound the thoracic cavity, which contains the lungs,
heart, and large blood-vessels.

[Illustration: Fig. 15.
A representation of the pelvic bones. _e_. The
lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_.
The innominata. _4,4_. Acetabula.]

The _Pelvis_ is an open bony structure, consisting of the Os Innominata,
one on either side, and the Sacrum and Coccyx behind. The _Sacrum_,
during childhood, consists of five bones, which in later years unite to
form one bone. It is light and spongy in texture, and the upper surface
articulates with the lowest vertebra, while it is united at its inferior
margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal
column. In infancy it is cartilaginous and composed of several pieces,
but in the adult these unite and form one bone. The _Innominata_, or
nameless bones, during youth, consist of three separate pieces on each
side; but as age advances they coalesce and form one bone. A deep
socket, called the _acetabulum_, is found near their junction, which
serves for the reception of the head of the thigh-bone.

[Illustration: Fig. 16.
1. Portions of the backbone. 2. Cranial
bones. _4_. Breast-bone. _5_. Ribs. _7_. Collar-bone.
_8_. Arm-bone (humerus). _9_. Shoulder-joint.
_10, 11_. Bones of the fore-arm (ulna and
radius). _12_. Elbow-joint. _13_. Wrist-joint. _14_.
Bones of the hand. _15, 16_. Pelvic bones. _17_.
Hip-joint. _18_. Femur. _19, 20_. Bones of the
knee-joint. _21, 22_. Fibula and tibia. _23_. Ankle
bone. _24_. Bones of the foot.]

THE BONES OF THE UPPER EXTREMITIES are sixty-four in number, and are
classified as follows: The Scapula, Clavicle, Humerus, Ulna, Radius,
Carpus, Metacarpus, and Phalanges. The _Scapula_, or shoulder-blade, is
an irregular, thin, triangular bone, situated at the posterior part of
the shoulder, and attached to the upper and back part of the chest. The
_Clavicle_, or collar-bone, is located at the upper part of the chest,
between the sternum and scapula, and connects with both. Its form
resembles that of the italic letter _f_, and it prevents the arms from
sliding forward. The _Humerus_, the first bone of the arm, is long,
cylindrical, and situated between the scapula and fore-arm. The _Ulna_
is nearly parallel with the radius, and situated on the inner side of
the fore-arm. It is the longer and larger of the two bones, and in its
articulation with the humerus, forms a perfect hinge-