396 ZOOLOGY. 
lating surfaces, there being no ribs connecting with them. The vertebre 
which follow the abdominal ones are soldered together, and constitute the 
quadrangular ossa sacra, or sacrum, concave below and convex above. 
Their number is generally very restricted, and varies within narrow limits. 
The vertebral column terminates by the vertebre of the tail, which in their 
form and number differ greatly from the others. The first ones still possess 
the canal for the spinal marrow, but it vanishes gradually; and the last of 
the series consists of a cylindrical or prismatical body, more or less elongated, 
with rudimentary apophyses, or completely deprived of them. 
The ribs correspond in number to the dorsal vertebra ; they are elongated 
and curved cylindrical, prismatical, or compressed bones, without lateral 
processes, and never immediately connected with the breast bone below. 
The breast bone itself is composed of numerous cylindrical or compressed 
pieces, situated behind each other, with which the true ribs are united by 
means of cartilages. The others, or false ribs, are situated behind the 
breast bone; are always shorter, and are connected together by cartilagi- 
nous pieces. 
Most of the mammals are provided with four legs; the hind pair consist- 
ing of a thigh, a shank, a tarsus, and a foot; the fore pair, of an arm, a fore 
arm, a wrist,and a hand. The fore legs are generally shorter than the hind 
ones, and usually bent a little inwards; but, again, in some they are so 
short, and the hind so long, that when the animal attempts to walk on four 
legs, the anterior part of the body is much lower than the posterior part, 
even though the hind legs be considerably bent. Hence, it results that 
these animals, as kangaroos, &c., prefer jumping or running on the hind 
legs alone. The opposite development of the legs, or the presence of 
fore legs longer than the hinder, is observed in the Asiatic orang outang, in 
the long-armed monkey, the sloth, &c. In the bat, also, the fore legs, or 
rather the fingers, are very long, and between them and the hind legs the 
skin of the body is extended, so that by this means these animals can 
keep on the wing. In others the skin is less expanded, and serves only as a 
parachute, and not for the real act of flying. Mammals walk either on the 
toes alone, or else, as in the bear, on the entire sole of the foot. In the first 
case, the foot is generally long, and forms, with the lower end of the leg, 
the backward directed knee. 
The feet are usually directed forwards; only in the walrus and seal, and 
slightly in the bat, the hind feet are directed backwards. Some are pro- 
vided with short and broad fore feet, fitted for digging or scratching in the 
sand. 
The normal number of toes is five, viz. the thumb, the innermost toe; 
next to this, the indicator-finger; then the middle finger ; next to the latter, 
the ring-finger ; and finally the outermost, the little finger. But in several 
venera there is one finger wanting to the hind feet; or two are wanting; or 
we find four toes to the fore feet, and five behind; or four everywhere ; or 
four to the fore, and three to the hind feet; or two before, and four behind; 
or two before, and three behind. These toes, however, are not always 
developed ; as, for instance, in the case of a single hoof, or of two hoofs, 
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