the outermost incisor is short, the bones of the middle part of the leg are separate, and there are at least three toes to each foot.
The longest-known genus and the one containing the largest species is Anchitherium, typically from the Middle Miocene of Europe, but also represented by one species from the Upper Miocene of North America. The European A. aurelianense was of the size of an ordinary donkey. The cheek-teeth are of the type shown in a of figs. 1 and 2; the premolars, with the exception of the small first one, being molar-like; and the lateral toes (fig. 3, c) were to some extent functional. The summits of the incisors were infolded to a small extent. Nearly allied is the American Mesohippus, ranging from the Lower Miocene to the Lower Oligocene of the United States, of which the earliest species stood only about 18 in. at the shoulder. The incisors were scarcely, if at all, infolded, and there is a rudiment of the fifth metacarpal (fig. 3, b). By some writers all the species of Mesohippus are included in the genus Miohippus, but others consider that the two genera are distinct.
Mesohippus and Miohippus are connected with the earliest and most primitive mammal which it is possible to include in the family Equidae by means of Epihippus of the Uinta or Upper Eocene of North America, and Pachynolophus, or Orohippus, of the Middle and Lower Eocene of both halves of the northern hemisphere. The final stage, or rather the initial stage, in the series is presented by Hyracotherium (Protorohippus), a mammal no larger than a fox, common to the Lower Eocene of Europe and North America. The general characteristics of this progenitor of the horses are those given above as distinctive of the group. The cheek-teeth are, however, much simpler than those of Anchitherium; the transverse crests of the upper molars not being fully connected with the outer wall, while the premolars in the upper jaw are triangular, and thus unlike the molars. The incisors are small and the canines scarcely enlarged; the latter having a gap on each side in the lower, but only one on their hinder aspect in the upper jaw. The fore-feet have four complete toes (fig. 3, a), but there are only three hind-toes, with a rudiment of the fifth metatarsal. The vertebrae are simpler in structure than in Equus. From Hyracotherium, which is closely related to the Eocene representatives of the ancestral stocks of the other three branches of the Perissodactyla, the transition is easy to Phenacodus, the representative of the common ancestor of all the Ungulata.
See also H. F. Osborn, “New Oligocene Horses,” Bull. Amer. Mus. vol. xx. p. 167 (1904); J. W. Gidley, Proper Generic Names of Miocene Horses, p. 191; and the article Palaeontology. (R. L.*)
EQUILIBRIUM (from the Lat. aequus, equal, and libra, a
balance), a condition of equal balance between opposite or
counteracting forces. By the “sense of equilibrium” is meant
the sense, or sensations, by which we have a feeling of security
in standing, walking, and indeed in all the movements by which
the body is carried through space. Such a feeling of security
is necessary both for maintaining any posture, such as standing,
or for performing any movement. If this feeling is absent or
uncertain, or if there are contradictory sensations, then definite
muscular movements are inefficiently or irregularly performed,
and the body may stagger or fall. When we stand erect on a
firm surface, like a floor, there is a feeling of resistance, due to
nervous impulses reaching the brain from the soles of the feet
and from the muscles of the limbs and trunk. In walking or
running, these feelings of resistance seem to precede and guide
the muscular movements necessary for the next step. If these
are absent or perverted or deficient, as is the case in the disease
known as locomotor ataxia, then, although there is no loss of the
power of voluntary movement, the patient staggers in walking,
especially if he is not allowed to look at his feet, or if he is blind-folded.
He misses the guiding sensations that come from the
limbs; and with a feeling that he is walking on a soft substance,
offering little or no resistance, he staggers, and his muscular
movements become irregular. Such a condition maybe artificially
brought about by washing the soles of the feet with chloroform
or ether. And it has been observed to exist partially after
extensive destruction of the skin of the soles of the feet by burns
or scalds. This shows that tactile impulses from the skin take
a share in generating the guiding sensation. In the disease
above mentioned, however, tactile impressions may be nearly
normal, but the guiding sensation is weak and inefficient, owing
to the absence of impulses from the muscles. The disease is
known to depend on morbid changes in the posterior columns of
the spinal cord, by which impulses are not freely transmitted
upwards to the brain. These facts point to the existence of
impulses coming from the muscles and tendons. It is now
known that there exist peculiar spindles, in muscle, and rosettes
or coils or loops of nerve fibres in close proximity to tendons.
These are the end organs of the sense. The transmission of
impulses gives rise to the muscular sense, and the guiding sensation
which precedes co-ordinated muscular movements depends
on these impulses. Thus from the limbs streams of nervous
impulses pass to the sensorium from the skin and from muscles
and tendons; these may or may not arouse consciousness, but
they guide or evoke muscular movements of a co-ordinated
character, more especially of the limbs.
In animals whose limbs are not adapted for delicate touch nor for the performance of complicated movements, such as some mammals and birds and fishes, the guiding sensations depend largely on the sense of vision. This sense in man, instead of assisting, sometimes disturbs the guiding sensation. It is true that in locomotor ataxia visual sensations may take the place of the tactile and muscular sensations that are inefficient, and the man can walk without staggering if he is allowed to look at the floor, and especially if he is guided by transverse straight lines. On the other hand, the acrobat on the wire-rope dare not trust his visual sensations in the maintenance of his equilibrium. He keeps his eyes fixed on one point instead of allowing them to wander to objects below him, and his muscular movements are regulated by the impulses that come from the skin and muscles of his limbs. The feeling of insecurity probably arises from a conception of height, and also from the knowledge that by no muscular movements can a man avoid a catastrophe if he should fall. A bird, on the other hand, depends largely on visual impressions, and it knows by experience that if launched into the air from a height it can fly. Here, probably, is an explanation of the large size of the eyes of birds. Cover the head, as in hooding a falcon, and the bird seems to be deprived of the power of voluntary movement. Little effect will be produced if we attempt to restrain the movements of a cat by covering its eyes. A fish also is deprived of the power of motion if its eyes are covered. But both in the bird and in the fish tactile and muscular impressions, especially the latter, come into play in the mechanism of equilibrium. In flight the large-winged birds, especially in soaring, can feel the most delicate wind-pressures, both as regards direction and force, and they adapt the position of their body so as to catch the pressure at the most efficient angle. The same is true of the fish, especially of the flat-fishes. In mammals the sense of equilibrium depends, then, on streams of tactile, muscular and visual impressions pouring in on the sensorium, and calling forth appropriate muscular movements. It has also been suggested that impulses coming from the abdominal viscera may take part in the mechanism. The presence in the mesentery of felines (cats, &c.) of large numbers of Pacinian corpuscles, which are believed to be modified tactile bodies, favours this supposition. Such animals are remarkable for the delicacy of such muscular movements, as balancing and leaping.
There is another channel by which nervous impulses reach the sensorium and play their part in the sense of equilibrium, namely, from the semicircular canals, a portion of the internal ear. It is pointed out in the article Hearing that the appreciation of sound is in reality an appreciation of variations of pressure. The labyrinth consists of the vestibule, the cochlea and the semicircular canals. The cochlea receives the sound-waves (variations of pressure) that constitute musical tones. This it accomplishes by the structures in the ductus cochlearis. In the vestibule we find two sacs, the saccule next to and communicating with the ductus cochlearis, and the utricle communicating with the semicircular canals. The base of the stapes communicates
