The thoracic limb consists of four chief segments, viz., the shoulder
girdle, the arm, the forearm, and the forefoot or manus.
The =shoulder girdle= (Cingulum extremitatis thoracicæ), when fully
developed, consists of three bones—the =scapula= or shoulder-blade, the
=coracoid=, and the =clavicle= or collar-bone. In the domesticated
mammals only the scapula, a large, flat bone, is well developed, and the
small coracoid element has fused with it, while the clavicle is either
absent or is a small rudiment embedded in the mastoido-humeralis muscle.
There is therefore no articulation of the shoulder with the axial
skeleton.
The shoulder girdle is fully developed in birds and the lower mammals
(monotremata). In the higher mammals the coracoid is reduced to the
coracoid process of the scapula, and the development of the clavicle
is in conformity with the function of the limb. Thus in typical
quadrupeds, such as the horse and ox, in which the forelimbs are used
only for support and locomotion, the clavicle is absent. Other animals
which use these limbs for grasping, burrowing, climbing, etc. (_e.g._,
man, apes, moles), have well-developed clavicles which connect the
scapula with the sternum.
The =arm= (Brachium) contains a single long bone, the humerus or arm
bone.
In the =forearm= (Antibrachium) are two long bones, the =radius= and
=ulna=. These vary in relative size and mobility. In the horse and ox
the two bones are fused, and the lower part of the limb is fixed in the
position of pronation. The radius is placed in front and supports the
weight. The ulna is well developed only in its upper part, which forms a
lever for the extensor muscles of the elbow. In the pig the ulna is the
larger and longer of the two bones, but is closely attached to the back
of the radius. In the dog the ulna is also well developed and a small
amount of movement is possible between the two bones.
The =forefoot= or =hand= (Manus) consists of three subdivisions, viz.,
the =carpus=, =metacarpus=, and =digit= or =digits=.
The =carpus=, popularly termed the “knee” in animals, and homologous
with the wrist of man, contains a group of short bones (Ossa carpi).
These are typically eight in number and are arranged in two transverse
rows—a proximal or antibrachial, and a distal or metacarpal. The bones
of the proximal row, named from the radial to the ulnar side (_i. e._,
from within outward), are the =radial=, =intermediate=, =ulnar=, and
=accessory= carpal bones. The bones of the distal row are designated
numerically, in the same direction, as =first=, =second=, =third=, and
=fourth= carpal bones.
This nomenclature, introduced by Gegenbaur, and now used largely by
comparative anatomists, seems decidedly preferable to the variety of
terms borrowed from human anatomy and based on the form of the bones
in man. The following table of synonyms in common use is appended for
comparison. The Latin terms and abbreviated notations are given in
parenthesis.
Radial (Os carpi radiale, Cr) Scaphoid
Intermediate (Os carpi intermedium, Ci) Semilunar
Ulnar (Os carpi ulnare, Cu) Cuneiform
Accessory (Os carpi accessorium, Ca) Pisiform
First carpal (Os carpale primum, C1) Trapezium
Second carpal (Os carpale secundum, C2) Trapezoid
Third carpal (Os carpale tertium, C3) Os magnum
Fourth carpal (Os carpale quartum, C4) Unciform
The central carpal bone (Os carpi centrale) is omitted, since it is
not a separate element in the animals under consideration here.
The =metacarpus= contains typically five =metacarpal bones= (Ossa
metacarpalia I–V), one for each digit; they are long bones and are
designated numerically from within outward. This arrangement occurs in
the dog, although the first metacarpal is much smaller than the others,
and the second and fifth are somewhat reduced. Further reduction has
taken place in the other animals, resulting in the perissodactyl and
artiodactyl forms. In the horse the first and fifth metacarpals are
absent, the third is the large supporting metacarpal bone and carries
the single digit, while the second and fourth are much reduced. In
artiodactyls (_e. g._, ox, sheep, pig) the third and fourth are the
chief metacarpals and carry the well developed digits; they are fused in
the ox and sheep. The others are variously reduced or absent as noted in
the special descriptions to follow.
The fossil remains of the ancestors of the existing Equidæ illustrate
in a most complete manner the reduction which has occurred in this
respect. The earliest known ancestor of the horse, Eohippus or
Hyracotherium of the Lower Eocene, had four well developed metacarpal
bones, each of which carried a digit; the first metacarpal bone was
small. Eleven intermediate stages show the gradual evolution of the
race from this primitive animal, which was about the size of the
domestic cat. There is reason to believe that earlier forms had five
digits.
The =digits= (Digiti manus) are homologous with the fingers of man, and
are typically five in number. They are designated numerically from the
radial to the ulnar side, in correspondence with the metacarpus. The
full number is present in the dog. In the ox and pig the third and
fourth are well developed and support the weight, while the second and
fifth are reduced. The existing horse has a single digit, the third of
his polydactyl ancestors. The skeleton of each fully developed digit
consists of three =phalanges= and certain =sesamoid bones=. The =first=
or =proximal phalanx= (Phalanx prima) articulates with the corresponding
metacarpal bone above and with the =second= or =middle phalanx= (Phalanx
secunda) below. The =third= or =distal phalanx= (Phalanx tertia) is
inclosed in the hoof or claw, and is modified to conform to the latter.
The =sesamoid bones= (Ossa sesamoidea) are developed along the course of
the flexor tendons or in the joint capsules. Two =proximal sesamoids=
(Ossa sesamoidea phalangis primæ) occur at the flexor side of the
metacarpo-phalangeal joint and form a pulley for the flexor tendon. The
=distal sesamoids= (Ossa sesamoidea phalangis tertiæ) are similarly
placed between the deep flexor tendon and the joint between the second
and third phalanx; they are absent in the dog, which has a small
sesamoid on the extensor side of the metacarpo-phalangeal joints, and
often at the proximal interphalangeal joint also.
Numerous cases are recorded of the occurrence of supernumerary digits
(hyperdactylism) in the horse and other animals. In some pigs, on the
other hand, the two chief digits are fused, and the condition appears
to be inherited.