All these Monkeys going very readily on all-fours have several interesting modifications of the structures observed in the climbing Monkeys, but of course their general construction is the same. They have not, however, the pouched stomachs of the Semnopithecus, and their nearest resemblance is to the African Guenons. Like all the Monkeys which are lower in the animal scale than the great man-shaped Apes, the Macaques have narrow wrists, long linger bones, and a short and backwardly-placed thumb. There are nine bones in the wrist. The hip-and haunch-bones are long, and the first are hollowed out, and their direction refers to the method of progression on all-fours, and their general appearance is rather that noticed in the regular four-footed beast of prey, and they differ much in breadth relatively to those of man. The length of the tail depends upon the number of the tail-pieces, or vertebra, and upon their size. In the Gibraltar Ape there are only three of these caudal vertebræ, but in the Bhunder there are fifteen and sometimes eighteen in the tail, which measures nine inches, whilst in the Pig-tailed Inuus there are seventeen. It appears that some of the long-tailed kinds have no more vertebræ than the others, but that the diminished length is due to their shortening. The long and middle-sized tailed kinds have chevron-or Y-shaped bones under the tail, and the nature of these has been explained already. Living upon a great variety of food, and using their jaws with rapidity, these Monkeys are furnished with a curious modification of a muscle, which exists in man and the higher Apes. There is in these a muscle on each side of the throat, which draws the chin down, or, in other words, helps to open the mouth. It is called the two-bellied, or digastricus muscle, as it has two muscular masses--one attached to the lower jaw, and the other to the lump of bone behind the ear--and they are united by a thin tendon. This tendon is attached to the side of the bone at the base of the tongue, or os hyoides, and it passes through a loop of a muscle which passes from the ear-bone (_styloid process_) to the os hyoides. The muscle acts as follows:--When the mouth is to be opened after swallowing, the base of the tongue-bone is pulled down by a muscle which comes from the breast-bone to it, and then the front belly, or muscle of the digastricus, pulls from the base of the tongue against the lower jaw and drags it open. But when the muscle relaxes, and the jaw is shut preparatory to swallowing, the digastricus begins to assist in this operation. In swallowing, the base of the tongue is drawn upwards towards the roof of the mouth, and the back and front bellies of the muscle now under consideration drag on their fixed tendon, and straighten, so as to assist in this. In the Macaques, this tendon is replaced by muscular bands, and greater vigour is given to the muscle, so that the jaw is pulled at more rapidly, and the tongue is elevated with energy.