afford a firm fulcrum to muscles passing from the caudal vertebræ to the pelvis and hind-limbs: the short fore-limbs are tucked up to the chest so as to offer the smallest surface to the air, and the animal progresses in a series of bounds by simultaneous movements of the hind-limbs.

The Rabbit, in moving slowly, advances the fore-feet two or three steps alternately. The body being thus elongated, the hindlegs are suddenly extended and drawn forward simultaneously: it thus, as it were, walks with the fore-legs, and leaps with the hind. The Hare is under disadvantage with its long hind-limbs in running down-hill, owing to the great inclination of the axis of the trunk to the plane of motion, and it usually zigzags as it descends; but it gains proportionally in the ascent, and its speed on level ground, through the size and strength of the chief pelling limbs, is very great. The degree of flexion of the trunk accompanying the movements of these and other quadrupeds is indicated by that in which the neural spines converge toward the single vertical one marking the centre of motion, and it is commonly greatest in the unguiculate quadrupeds.

pro

The verticality of the long and narrow tarsus and metatarsus producing the digitigrade' character of the type Carnivora, combines with the geometrical and physical relations of the other parts of the limbs to give them their superior speed and agility. In the Dogs and Cats the oblique scapula, being unfettered by bony (clavicular) connection with the sternum, enjoys the freedom of rotation which characterises it in the swift Ungulates. The humerus in the Lion (vol. ii. fig. 337) has its axis directed downward and backward, forming with that of the scapula an angle of 110°. The olecranon projects so far behind the axis of rotation in the elbow-joint as to constitute a powerful lever for the extensors of the fore-arm. The hind-limbs are longest, and the bones are inclined more obliquely to each other than in the fore-limbs, subserviently to elasticity and power in springing. The calcaneum is produced on the same principle as the olecranon, but forms the more powerful lever of the two. The last perfection is given to the limbs of Carnivora by the modifications of the toes of Felines, whereby their tread is noiseless, and the claws exempt from the wear and tear of progressive motion. It is effected by a joint allowing the ungual phalanges to be brought in extension above the middle phalanges, elastic ligaments being adjusted to keep the joint so extended, and by a thick cushion of soft elastic substance beneath the joint or parts of the phalanges transmitting the superincumbent weight to the ground.

In the toes of the fore-foot the last phalanx is retracted on the ulnar side of the second phalanx. The principal elastic ligament arises from the outer side and distal end of the second phalanx, and is inserted into the upper angle of the last phalanx: a second arises from the outer side and proximal end of the second phalanx, and passes obliquely to be inserted at the inner side of the base of the last phalanx: a third arises from the inner side and proximal end of the second phalanx, and is inserted at the same point as the preceding.

36

The tendon of the flexor profundus perforans' is the antagonist of these ligaments. The toes of the hind-foot are retracted in a different direction, viz. directly upon, and not by the side. of, the second phalanx; and the elastic ligaments are differently disposed. They are two in number, arise from the sides of the second phalanx, and converge to be inserted at the superior angle of the In fig. 36, a is the pair of elastic ligaments; b, the tendon which pulls out and works the claw; c, inelastic ligament continued from the extensor' tendon, which is mainly inserted into the second phalanx.'

6

The main purport of the modifications of the motory system in Quadrumana is to make them climbers. By the developement and direction of the hallux the hind-foot is converted into a hand, with unusual power of prehension, especially in the Gorilla; the joint of this hand is so modified as to give it a free motion excentric to the axis of the leg, whereby its outer edge is applied to the ground; the whole hind-limb is shortened, disproportionately so in the best climbers (vol. ii. fig. 180), in which also the hind-limb may be unfettered, for its acts of manipulation, by the absence of the ligamentum teres' of the hip-joint (Pithecus). The length of the iliac bones relates to elongation of the muscles for rotating the hind-limb and hand more quickly and through greater spaces. Correlatively, the scapular arch approximates to the condition of the pelvic one by the extension of complete clavicles to the manubrium, and the head of the humerus is received into a deeper and more secure socket than in Bimana. This is well exemplified in the long-armed Gibbons, which enjoy the peculiar mode of locomotion called 'brachiation.' The body is set into pendulous vibration by the action and reaction of the

The dissections of the Lion's foot showing the above-described modifications of the elastic ligaments are Nos. 287A and 288A, Physiol. Series, vol. i. xx.

muscles of one arm and of the trunk, the force finally attained and the swing being such as to propel the animal some distance through the air; a bough is seized by the opposite out-stretched arm, and the momentum is applied in aid of a repetition of the action to gain a longer launch. I have myself witnessed, in the London Zoological Gardens, an aerial leap of upwards of fifteen feet so effected by the long arms of a captive Hylobat. M. Duvaucel, observing them in their native forests, testifies to their passing through a distance of forty feet from bough to bough. Mr. Martin, when curator of the Zoological Society's Museum, watching the same female Hylobates agilis which had been the subject of my own study of the brachiating mode of motion, states that, a live bird being set at liberty in her presence, she marked its flight, made a long swing to a distant branch, caught the bird with one hand in her passage, and attained the branch with her other hand, her aim both at the bird and the branch being as successful as if one object only had gained her attention.'I

In most of the Platyrhine monkeys the tail is prehensile, and becomes, in Ateles more especially, a fifth independent organ of grasping.

In ordinary progression on the ground the Quadrumana move as quadrupeds; but the higher tailless Catarrhines (Apes), instead of setting the palm or outer margin of the fore-hands, like the inferior families, to the ground, apply the back of the second phalanges of the flexed fingers, the skin covering which has a broad and thick callosity, whence these apes are sometimes called collectively, knuckle-walkers.' The longer-armed kinds, in slow movement, support the body upon the knuckles, as upon a pair of crutches, and swing the hind-limbs forward between them. In more rapid movement they sway the trunk and hind-limbs in a sort of sidelong sweep, progressing by a kind of shambling amble. The tracks of the Gorilla show this to be the habitual mode of progression along the ground. Station or motion on the lower limbs only is shown to be difficult by its awkwardness and the shortness of time during which it can be maintained. The walk is a waddle from side to side, the huge superincumbent body being balanced by swinging movements of the long arms, or by clasping the hands behind the head. When so pursued as to be driven to stand at bay, the Gorilla, like the plantigrade Bear, raises himself on the hind-hands, so as to have his powerful arms and fists free for the combat.

The Bimana are as expressly adapted to station and movement

on the ground as are the Quadru-
mana to climbing in the forest. There
is no known connecting link between
the lowest variety of Man and the
highest species of Ape. No animal
is served by arms, at once so large
and variously flexible and applica-
ble as Man; in none are the termi-
nal divisions of the limb so distinct
in their power and adaptibility.'
The mechanism of the vertebral
column and limbs which makes
Man a' plantigrade biped,' and the
only one in the Animal Kingdom,
is as perfect in the Mincopie,2
Australian, or Boschisman, as in
the most advanced member of the
white race.
The locomotive frame
of any variety would equally serve.
as the subject of such elaborate
analyses of the mechanical condi-
tions of standing,' walking,' run-
ning,' 'leaping,' &c. as have been
given by Borelli,3 Barthez, Rou-
lin, Gerdy, and W. & E. Weber,7
to whose works, and especially the
latter, the reader is referred for
this interesting branch of Animal
Mechanics.

5

=

=

Figure 37 exemplifies a Man stooping with a load, and sustained in that position by the glutei, f, the quadriceps femoris, y, and the gastrocnemii, l. If the weight r be 120 lbs., that of the bearer 150 lbs., and if the line r s be the direction of the force of gravity cutting the femur and tibia in c and x, the centre of gravity of the Man being at b, and the common centre of gravity of the Man and his load at a, then the weight of the Man from the head to b will be 150 lbs. 75 lbs., and that of the section b to c, by supposition, = 47; therefore the weight of the arc a b c = 75 + 47 = 122, also by supposition the section c v x = 20, and consequently the whole arc a b v x = 142; the distances of the directions of the muscles from the axes of the joints to the distances of the line of gravity are, according to Borelli, in the following ratio,the distance fb is to the distance m b as 1 is to 8; ov is to tv as 1 to 6;k d is to pd as 1 to 3; and tv to bm as 3 to 4; hence he derived certain proportions, from which he estimated that the extensor muscles of the leg, to sustain this weight, exerted a force =6032 lbs., being more then fifty times the weight.

38

CHAPTER XXVIII.

NERVOUS SYSTEM OF MAMMALIA.

§ 203. Myelon. The myelon in Mammals, as in Birds, quits, in the course of developement, the hinder part of the neural canal, moving and concentrating forwards, and leaving the concomitantly elongated roots of the nerves, between their places of exit at the intervertebral foramina and their places of attachment to the myelon, as an indication of the primitive extent of the nervous axis.

It is remarkable that the Monotrematous order, so restricted in its representative genera, should present the two extremes of this developemental difference in the length of the myelon. The Ornithorhynchus hardly departs from the condition of the lizard, the myelon extending into the sacrum, and having the intravertebral nerve-roots limited to the short canal of the caudal region; whilst in the Echidna, fig. 38, the myelon moves forward to the middle of the dorsal region, d, where it ends in a point, and leaves all the canal behind occupied by the elongated nerve-roots and shrunken emptied myelonal sheath, answering to the cauda equina' and ' filum terminale' of anthropotomy, but of extraordinary length.

In the Ornithorhynchus the myelon fills closely the neural canal: it is thickest at its commencement and at the lower two-thirds of the cervical region; it is more slender in the back, especially near the loins; it is slightly enlarged in the lumbar region, and gradually terminates in a point at the end of the sacral canal. The short and thick myelon of the Echidna presents the two usual enlarge

d

Brain and spinal chord,
Echidna, half nat. size.