134 



M A M M A L I A. 



peculiar twisting motion in the elbow joint which 

 makes the bird's wing act to the greatest possible 

 advantage against the air. The bones in the ex- 

 tremity of this organ of flight are indeed the very 

 reverse of those in the bird's wing, for while the 

 terminal bones of that wing are soldered together, so 

 that it strikes the air in one mass, those of the bat's are 

 spread out, and the stroke upon the air is divided into 

 as many parts as there are ringer bones supporting it. 

 The production of the wing membrane too, which 

 is often continued, with more or less interruption in 

 the middle part, to the hind legs, and sometimes for- 

 ward to the ears, and also backward so as to include 

 the tail, does not allow the same freedom on the first 

 or shoulder joint of the wing as birds possess ; and 

 from these circumstances the style of their flight bears 

 more resemblance to the flutter of the larger insects 

 than to the clean and effective strokes by means of 

 which an air bird dashes along. Some details of the 

 different parts of their structure will be found on 

 referring to the article BAT in this work ; but in 

 order that we may better see the contrast between 

 them and the other mammalia, as well as between 

 flying mammalia and birds, there are one or two par- 

 ticulars to which it is necessary to advert. The 

 flexible spine, the weight of the hind legs and the 

 tail, and also some physiological considerations, ren- 

 der it necessary that the wings of the bats should in 

 part support the posterior extremities by absolute 

 connexion with the moving part of the wing, and not 

 through the medium of its insertion ; and there is 

 this reciprocal advantage in the membranes which 

 extend from the wing, that they steady it, and pre- 

 vent such an action of the large blade-bones against 

 the ribs as would otherwise take place in consequence 

 of the weight of the body being less completely 

 borne upon the sternum than it is in birds. Alto- 

 gether, the wings of bats are to be considered as 

 peculiar modifications of the anterior extremities 

 of mammalia for a specific purpose, and not as 

 bearing any structural analogy to those of birds. 



In the posterior extremities, and these partake but 

 little in the motion of the wings, though they yield 

 to that motion, there is perhaps a still greater devia- 

 tion from the usual form of the mammalia. These 

 legs have, in the hip joint or articulations of the 

 femur, a sort of half revolution outwards and inwards 

 upon an axis, so that when the legs are bent, the toes 

 and the legs are directed towards the belly, while the 

 thigh is directed backwards. This arises in part from 

 the backward situation of the cavities into which the 

 heads of the femurs are inserted. This moving ol 

 the knee joint backward, while the foot and leg are 

 brought obliquely forward, enables the first of these 

 joints to tighten the posterior part of the membrane 

 into a parachute, while the contrary flexure of the 

 foot forms a cavity at each side, and so makes it 

 take a better hold on the air. This species of 

 resistance will be better understood by those unac- 

 quainted with the mechanical properties of the air, 

 by noticing how much more speedily a little cap 

 made of paper falls to the ground when the projecting 

 side of it is turned downwards, than when the hollow 

 is in that direction. This converting of the hinder 

 part of the membrane into a parachute, which is sup- 

 ported by the resistance of the air under it, owing to 

 its escape being prevented by the lateral bending down- 

 ward, compensates in some sort for the imperfections 

 of those parts of the membrane which are moved by 



;he arms, and perform the active part of the flying. 

 The membranous wings do not, like the feathered 

 wings of birds, admit the air to pass through them 

 when they are raised, and in consequence of this, 

 excepting in so far as the rising wing is contracted 

 in size by the flexure of the joints, as compared 

 with the descending wing, which takes effect 

 with the joints extended, there is not much in 

 the structure of the bat's wing to make the one 

 side of it offer less resistance to the air than the other 

 does ; and this is the reason of the great rapidity 

 with which the wings are moved both ways, the 

 parachute portion holding on until the moving part has 

 gained its elevation, and is ready to descend again. 



The whole structure of the bats, and more parti- 

 cularly of those which % high and with considerable 

 rapidity, is so formed as to give them a very great 

 horizontal surface in proportion to their weight and 

 length. The wings and continued membranes are 

 not only very large, but the body is short and broad, 

 which both increases the horizontal surface, and 

 affords more room for the pectoral muscles. The 

 posterior part of the body is made slender, and all 

 the parts with which the anterior of the body can 

 be loaded, consistently with the functions of mam- 

 malia, are situated there ; for instance, the mamma? 

 of the females are pectoral, and when they carry 

 their young on their flights, these cling to the breast. 

 The wings of bats are not capable of bearing nearly 

 the same exposure to the atmosphere and the sun as 

 the wings of birds. The membranes possess a good deal 

 of sensibility, and their motion would be rendered 

 much more stiff and difficult if they were to become 

 dry. Hence bats, at their regular hours of appearance, 

 are never seen on the wing when the wind blows 

 strongly; nor, notwithstanding their love of shade and 

 humid air, especially in hot countries, are they ever 

 seen during heavy rain ; it is probable indeed that 

 the violent rains which prevail in those tropical coun- 

 tries would beat them to the ground, just as a tropical 

 hurricane would deprive them of all command of the 

 air, and drive them before it like chaff. A high tem- 

 perature appears, however, to be essential to the 

 proper working of their very curious structure, for 

 they hybernate not merely in the cold latitudes, but 

 in those which are moderately temperate. They are 

 creatures of gloomy caves and stilly twilights ; at 

 which times they beat the air with great industry, 

 and most of them are very ravenous, even the ones 

 which feed upon vegetable substances. 



The peculiar flexure of the legs to which we have 

 alluded, which turns the toes of the hind feet inward, 

 is of great advantage to them in their manner of 

 resting, which is usually that of suspending them- 

 selves upon the sides and the inequalities of the roofs 

 of those caves, ruined temples, deserted houses, and 

 other places where they take up their abode. The 

 crooked claws upon the thumbs of the wings take 

 hold with their points so situated, as that the axis of 

 each is directed nearly toward the centre of gravity 

 of the body, and thus the weight draws them, or 

 tends to draw them inwards as well as downwards ; 

 the crooked claws again on the hind feet are by the 

 position which they take when bent, so placed, as 

 that they are drawn, or tend to be drawn, outward 

 and downward ; and as they have the line of their 

 action also directed nearly from the centre of gravity 

 of the body, the weight of the body tends to make each 

 hind claw pull so as to tighten the fore claw on the 



