The upper arm bone, or humerus, like most other bones of the extremities, has been greatly modified by the habits of the different reptiles. In running and climbing reptiles it is always slender, while in burrowing reptiles it is short and stout and much expanded at the extremities, like the humerus of the mole among mammals. And we shall also see how greatly modified it was among the swimming reptiles. The humerus of flying reptiles has an enormous process on the side, corresponding to the attachment of the deltoid muscle. The head of the humerus, for articulation with the glenoid cavity of the scapula, is rounded in all reptiles, except the pterodactyls, and the articulation is always at the extremity. At the lower extremity the protuberance at the outer or radial side is called the ectocondyle; that on the inner or ulnar side, the entocondyle. Between the two at the end are the articular surfaces for the radius and ulna, the capitellum and trochlea. A little above each of these condyles there is usually, on one side or the other or on both, a foramen or hole for the passage of arteries or nerves. That on the inner side, which is characteristic of all early reptiles and of many mammals, is called the entepicondylar foramen; that on the outer side, the ectepicondylar foramen; the latter is present in the lizards, and both are found in the tuatera and some of the early reptiles. [Illustration: FIG. 21.—Anterior extremity of _Ophiacodon_.] The radius and ulna are always distinct bones in reptiles, and always freely movable on each other; they are usually shorter than the humerus, but in some springing and climbing reptiles they are quite as long. The carpus or wrist of reptiles consists primitively of eleven distinct, irregularly shaped bones, which articulate more or less closely with each other in three rows. Those of the first row, all true carpals, are known usually as the radiale, intermedium, ulnare, and pisiform, corresponding quite with the bones of the human wrist known as the scaphoid, lunar, cuneiform, and pisiform. The second row has but two bones, on the radial side, known as the centralia; while the third row has a bone to correspond to each of the metacarpals, five in number, and collectively known as the carpalia. Some or indeed all of these bones may be either absent or unossified, that is, remaining through life as nodules of cartilage. Seldom, however, are there less than nine bones in the carpus of reptiles. The metacarpals, like the digits, primitively were five in number, and seldom are there less, though the fifth is sometimes lost, and rarely also the first. They are more or less elongate bones, increasing in length from the first to the fourth, with the fifth usually shorter. The first and the fifth are usually more freely movable on the wrist than are the other three.