180 



SPECIAL PHYSIOLOGY. 



quadrupeds, moving like the horse, zebra, and donkey, there is no clavicle, as 

 in the rhinoceros, elephant, hippopotamus, hog, ox, giraffe, camel, stag, ante- 

 lope, goat, sheep, and other Kuminants. In all such animals, it is to be noted 

 that the anterior limb is used exclusively for purposes of locomotion, or for 

 those of offence or defence ; but it has no prehensile faculty. In a great num- 

 ber of the four-footed Mammalia, however, the anterior limb has a prehensile 

 character, or is used in some special manner adapted to the habits of the 

 animal, in addition to its mere locomotive function ; in such cases, a collar- 

 bone, more or less perfectly developed, is found. Thus, in the Carnivora, in 

 which the anterior limb is used in striking or seizing the prey, a short, imper- 

 fect collar-bone is found, which is smaller in the dogs and larger in the cats ; 

 in the seal tribe it is absent. A slender collar-bone exists in certain Kodentia 

 (as the rabbit, hare, and rat), which use their anterior limbs in scraping and 

 burrowing ; and in the squirrel, which can climb, seize, and hold nuts and 

 other food in its fore-paws, the clavicle is well developed. In the Cheiroptera 

 (bats) the anterior limbs are organized for flight, and also have a clavicle, 

 which is long, strong, and bent. This bone is likewise present, and strongly 

 developed, in the burrowing Insectivora, being short, broad, and cubical in the 

 mole ; it is also large in the Edentata (armadillo, ant-eater). Even in the low 

 Marsupials (as the kangaroo), which have considerable prehensile power in 

 their comparatively small fore-limbs, the clavicle is present. In this creature, 

 the quadruped mode of progression is only occasionally employed ; but its more 

 active movements consist of powerful leaps, which it performs by the sudden 

 extension of its large hinder limbs, and by the flexion of its powerful tail, the 

 muscles of both of which parts are enormously developed ; there is a sharp claw 

 on the large fourth toe, which is used to tear open the flank of an attacking 

 animal. With the kangaroo, a very common attitude is a semi-erect position, 

 maintained by resting upon the hinder limbs and tail. The squirrel, likewise, 

 uses its large bushy tail, both as an organ of support in sitting, and also in 

 leaping. 



A still further deviation from the ordinary mode of progression on land, in 

 Mammalia, is witnessed in the seals, which animals,, when out of the water 

 and moving over the ground, accomplish this, partly by a feeble and awkward 

 motion of the anterior paddle-like limbs, and partly by a wriggling motion of 

 the hinder portion of the body. The Cetaceans probably flounder still more 

 helplessly upon the ground. 



In Birds, the attitude upon, and mode of progression over, solid surfaces, 

 is biped, or as in the case of many birds, whilst resting or actually sleeping, 

 the standing position is accomplished upon one leg only or is uniped. The 

 centre of gravity being situated very far forward in the body of a bird, owing 

 to the great size of its pectoral muscles and wing bones, the trunk is usually 

 held, in the standing position, inclined very little from the vertical direction, 

 so as to bring the weight more over the base of support in the soles of the feet. 

 The lumbar and dorsal regions of the vertebral column are very strong, and 

 exhibit but little power of bending. To aid in supporting the weight, this base 

 is extended forwards by the elongation of the phalanges of the toes ; and to 

 give the necessary rigidity to the trunk, the pelvis and lumbar region are 

 solidified together, whilst the dorsal vertebrae are capable only of comparatively 

 slight motion, the respiratory movements being performed rather by the descent 

 of the sternum, than by any expansion of the ribs ; the neck also, which is 

 usually long in birds, to suit the prehensile character of the bill, is capable 

 of being folded back over the trunk, so as to bring its weight over the base of 

 support ; the wings, as usually folded in walking, also serve to transfer weight 

 to the hinder part of the body. In standing on a level surface, the weight of 

 the bird is transmitted through the elongated metatarsal bones, and is then 

 distributed through the several toes, the length of which varies in different 

 species, according to the hardness or softness of the ground on which, in ac- 

 cordance to its habits, it usually has to run. In other instances, the foot is 

 better suited for grasping more or less prominent surfaces, or even the trunks 

 and smaller boughs of trees, bushes, or other plants ; in which case, the toes 

 are shorter and stronger. In some scansorial tribes or climbing birds, one of 

 the three toes, which in other birds are turned forwards, is, at the will of the 

 bird, or else permanently, turned backwards, so that there are two toes in front 



