On looking at the arm-bone, which, being connected to the shoulder by a joint, has much to do with the act of climbing and striking, it will be noticed that it greatly resembles that of man in shape, but is longer, stouter, and clumsier. The joint is nearly in the shape of a rounded knob, and the corresponding depression or cup on the blade or shoulder-bone into which it fits, is an oval and concave surface, and they are kept close together by a kind of capsule which stretches from one bone to the other and encloses the joint. Perfect freedom of movement is insured by the bones being covered with glistening cartilages, and a delicate and moist membrane, and the motion from the shape of the apparatus is almost equal to that of a chandelier where there is what is called a cup-and-ball joint at the ceiling. It has already been noticed that muscles are attached to the blade-bone and to the arm-bone below the joint, and that, this being movable, when they contract they move the arm, and the instance was given of the action of the deltoid muscle in raising the arm. In the Gorilla, this great muscle reaches lower down than in man, and there is a very strong mark in the shaft of the bone for its insertion. This gives the muscle greater play than in us, and enables it to lift, more slowly perhaps, but more efficiently, for the arm-bone between the joint and the place where the muscle is attached, is the long arm of a lever which is shorter in man. Below the globular head of the arm-bone is the shaft or cylindrical part of the bone which gives origin to the three-headed muscle called triceps, and is covered by the two-headed one (biceps) already mentioned, besides the deltoid. A deep groove allows one of the ends or heads of the biceps to pass along and slide over the joint and to reach the shoulder-blade. The shaft as a whole is more or less cylindrical, with a slight angular outline, the angles being projections of bone which strengthen the whole, besides giving attachment to muscles; the cylindrical shape is the best for strength and lightness, and these properties are increased by the adoption of a plan which engineers have long since unwittingly copied. The shaft is hollow, and is cellular at both ends, solid bone covering the outside, conditions which oppose fracture, and produce increased strength, indeed greater strength and lightness than a solid bone would have. Below the shaft is an expansion, on which are placed the surfaces for the jointing on of the two bones of the fore-arm, and the bone is especially in old Gorillas perforated there, a condition seen in some very old human bones. There is an important point in the relative length of the upper arm-bone, and the bones of the fore-arm in the Gorilla, in other Apes, and in man, for in this great Ape and in us the humerus is longer than the others, and in the Chimpanzee they are almost equal, whilst in the rest of the Monkeys they are very unequal,