280 MAMMALIA. 



the sacrum, and is closed ventrally by the symphysis of the pubis 

 and sometimes also of the ischium. The appendages articulated to 

 the pectoral and pelvic girdles are considerably shortened in the 

 swimming Mammalia, and either constitute, as in the Cetacea, flat 

 fins, the bones of which are immovable upon one another (in the 

 Sirenia there is a joint at the elbow), and in which there are a great 

 number of phalanges, or, as in the Pinnipedia, have the form of 

 fin-like legs, which can also be used in locomotion on land. In the 

 Cheiroptera (Bats), the anterior legs present a large surface in con- . 

 sequence of an expansion of .the integument (patagium) uniting the 

 fore-limbs with the sides of the body, and extended between the 

 elongated fingers. The fins of the Cetacea and the wings of the 

 Cheiroptera are, with the exception of the thumb of the latter which 

 projects from the patagium and bears a claw, without nail -like 

 structures. 



In the land Mammalia the extremities present considerable varia- 

 tions both in their length and special structure. The length of the 

 tubular humerus in general varies inversely with that of the metar- 

 carpus of the anterior extremity. The radius and ulna in the fore- 

 limb and the tibia and fibula in the hind-limb are almost always 

 longer than the humerus and femur respectively. The ulna forms 

 the hinge- joint of the elbow, and is prolonged at this point into a 

 process called the olecranon ; the radius, on the other hand, is con- 

 nected with the carpus, and can often be rotated round the ulna 

 (pronation, supination) ; in other cases it is fused with the ulna, 

 which then constitutes a rudimentary, styliform rod as far as the 

 articular process. In the hind-limb the knee-joint projects forwards, 

 and is usually covered by a knee-cap, the patella] the fibula is some- 

 times (Marsupials) movable on the tibia, but as a rule these two 

 bones are fused, and the fibula which is placed posteriorly and 

 externally is usually reduced. The variations in the terminal parts 

 of the limbs are far more striking (fig. 670). The number of digits 

 is never greater than five, and is often less. The digits disappear in 

 ^the following order: firs',, the inner digit or thumb (digit No. 1), 

 which is composed of i.wo phalanges, becomes rudimentary and 

 vanishes; then the small 'outer digit (digit No. 5) and the second 

 inner digit (digit No. 2) are reduced, sometimes remaining on the 

 posterior surface of the foot (Ruminants) as small accessory claws 

 which do not reach the ground, or totally vanish. Finally the 

 second external digit (No. 4) is reduced or absent, so that the middle 

 digit alone remains for the support of the limb (horse). 



