Man walks on the sole of the foot. Some other animals (dog, cat) on the toes; others again (horse, ox, pig), on the tips of the toes. In the last case there is not simply a horny structure (nail or claw) on the upper side of the toe, but a hoof sheathing the whole of its tip. In many animals the thigh and upper arm are drawn close up to the body, so that the limbs appear quite different from those of man. (Compare Fig. 2 with Fig. 3.) [Illustration: FIG. 3.—Skeleton of an Ox. I. _Skull_: 1, Frontal bone, with horn cores, _a_; 2, temporal bone; 3, malar or cheek-bone; 4, maxillary bone; 5, lachrymal bone; 6, nasal bone; 7, premaxillary bone; 8, lower jaw; 9, orbit; 10, occipital bone. II. _Neck and Trunk_: _H_, 7 cervical vertebræ; _R_, 13 thoracic vertebræ; _L_, 6 lumbar vertebræ; _K_, sacrum; _Su_, caudal vertebræ; _C_, 13 pairs of ribs; _D_, sternum. III. _Fore Limbs_: _Sc._, scapula; _A_, humerus; _S_, radius; _E_, ulna; _U_, carpus; _M_, metacarpals; i., ii., iii., phalanges. IV. _Hind Limbs_: _B_, hip-girdle, _a_, ilium, _b_, ischium; _F_, femur; _P_, patella; _T_, tibia; _Sp_, tarsus; _M_, metatarsals; i., ii., iii., phalanges. ] The bones are usually surrounded by flesh. This consists of a number of different pieces united together by a delicate, elastic, fibrous mass (connective tissue). The different pieces are termed _muscles_, each of which is again made up of a large number of muscle-fibres, all taking a longitudinal direction. Each fibre can contract, and a muscle becomes shorter and thicker by simultaneous contraction of all its fibres. The contraction and subsequent relaxation of muscles move other parts. There are some muscles, the _hollow muscles_, which surround a cavity, and by their contraction propel the liquid or solid substances found in their cavity. The heart, for example, is a large muscle of this sort, serving to propel the blood, while the hollow muscular coat of the gut moves on the contained food. Other muscles are fixed by their ends to other parts of the body, which they move by their contraction. We distinguish between _dermal muscles_ and _skeletal muscles_, attached respectively to the skin or by one end to an integumentary structure (hair, feather, scale), and to parts of the skeleton. The animals which are devoid of any internal skeleton, the _invertebrates_ (_i.e._ all animals except vertebrates), naturally possess no skeletal muscles. Examples of _dermal muscles_ are those by means of which a bird erects its feathers (tail-coverts of peacock!), and those which enable a hedgehog to roll itself into a ball and stick out its spines. Each end of a _skeletal muscle_ is connected with a bone. If such a muscle contracts the more easily movable bone is drawn towards the less easily movable one (Fig. 4). In order that the bones may be movable upon one another they are united together by joints.