The radius, like the other bones of the an terior extremity, is remarkably elongated, and rather robust. The absence of rotation in the forearm of these animals forms an admirable adaptation to their habits. Not only would the pronation and supination of the hand be wholly useless to them, but at every impulse of their flight such a motion would deprive the whole limb of its resistance to the. air, or it would require the constant exertion of ,such a degree of antagonizing muscular force to prevent it, as would be incompatible with the essential structure of these organs of flight.
The carpus is of a very peculiar structure. The first series of bones consists but of two ; one very large, on which the radius rests, and which is probably formed of the three outer bones, the scaphoid, the semilunar, and the cuneiform bones ; the other extremely small, which is undoubtedly the pisiform, on the ulnar side.
The second series consists of the four bones of which it is usually constituted.
The metacarpal bones and phalanges of all the fingers excepting the thumb are extremely elongated. They extend outwards and down wards in a slightly curved direction to the margin of the flying membrane, the second finger being the shortest and extending to the upper angle of the outer margin, the third, fourth, and fifth to the inferior margin of the membrane. There is a slight enlargement at the articulation of the metacarpal bones with the phalanges; but otherwise these bones are extremely slender and cylindrical. The thumb is of no extraordinary length, and the ultimate phalanx is hooked and sustains a nail, by which the animal is enabled to climb on any rough perpendicular surface, or to suspend itself from some projecting part.
The pelvis is remarkably strait, rather elon gated, somewhat wider inferiorly. The ilia are narrow and elongated ; the ischia in several species, instead of receding from each other, approach so that their tuberosities touch each other, and in some instances come in contact with the coccygeal bones. In some species of Pteropus, the anterior portion of the ossa pubis, instead of meeting at the median line, recede more or less from each other, and the space is filled by ligament. In some species there is a sexual difference in this respect; the two pubic bones being in contact in the male and sepa rated in the female.
The sacrum and the ilia are connected by absolute bony union at an early period. The femur is of moderate length, slender and cy lindrical. It is turned outwards and upwards, so that the side which is usually anterior is directed nearly backwards. The tibia offers no peculiarity which requires particular notice.
The fibula is exceedingly small, slender, pointed towards its femoral extremity, and has this singular peculiarity, that it does not rise to the head of the tibia. In other cases where this bone is defective, it is at its inferior ex tremity, but in the present case it is the supe rior portion which is wanting. As the femora are directed outwards, the leg-bones are in some measure turned round, so that the fibula are at the inner side of the tibia and a little behind them.
The foot of the Cheiroptera does not ex hibit the same deviation from the normal structure which we have seen in the hand. On the contrary, it is not extraordinarily developed, and the different parts of which it is composed are in the usual relative proportions.
The tarsus is composed of the usual bones. There is a peculiarity in the heel, however, which is worthy of notice. There is a long, slender, pointed, bony process from the pos terior part of the foot which is inclosed within the folds of the margin of the interfemoral membrane, and extends about half-way to the tail. Whether this process is a portion of the . os calcis, according to Cuvier, or a distinct bone according to Daubenton, it is perhaps difficult to decide; but the opinion of Meckel is probably the correct one, that it is nothing more than a development of the tuberosity of that bone, remaining disunited from its body.
Thenietatarsal bones are rather short, slender, and of nearly equal length.
The phalanges of the five toes are nearly equal, the inner toe reaching almost to the same length as the others, in consequence of the greater elongation of its first phalanx. The ultimate phalanges are furnished with booked nails, by which these animals constantly suspend themselves when at rest with the head downwards.
The whole of this structure is so perfectly adapted to the peculiar habits of the animals, as to require no comment. The great deve lopment of the ribs, sternum, and scapula, for the attachment of strong muscles of flight, the length and strength of the clavicle, the exten sion of all the bones of the anterior extremities, all admirably tend to fulfil their obvious end. The existence of a tail for the support and extension of the interfemoral membrane, which is found in the insectivorous Bats, compared with its absence or comparative inefficiency in many of the frugivorous, also points out an interesting relation to the different habits of the two groups, the former structure being calcu lated to afford a powerful and effective rudder in guiding their rapid and varying evolutions in the pursuit of their insect food.