646 MANUAL OF ZOOLOGY. 



The power of opposing the thumb to the other digits of the 

 hand is found only in Man, and in a considerable number of 

 the Quadrumana, but never so perfectly developed as in Man. 

 In Man only does this power attain its full perfection, and it 

 constitutes one of the most striking of the merely anatomical 

 peculiarities by which Man is separated from the Monkeys. 

 As, however, this feature is purely adaptive, and is really to be 

 regarded as of extremely small physiological value, we ought 

 to learn from this that the difference between Man and the 

 Quadrumana is to be sought in the mental powers of each, and 

 not in any merely structural character. 



Whilst the anterior limbs are never absent in any Mammal, 

 the posterior limbs are occasionally wholly wanting, as in the 

 Cetacea and Sirenia (in Halitherium, an extinct Sirenian, a 

 rudimentary femur exists). Generally speaking, however, the 

 posterior limbs are present, and the pelvic arch has much the 

 same structure as in Man. The two halves of the pelvis the 

 ossa innominata consist each of three pieces in the embryo 

 viz., the ilium, ischium, and pubes which meet to form the 

 cup-shaped cavity known as the " acetabulum," with which the 

 head of the thigh-bone articulates. In the adult Mammal 

 these three bones are anchylosed together, and the two ossa 

 innominata unite in front by means of a symphysis pubis, con- 

 stituted either by a cartilaginous union (synchondrosis), or by 

 merely ligamentous attachment. In some Mammals, however, 

 such as the Mole, and many of the Bats, the pubic bones 

 remain disunited during life. As a rule, also, the ossa inno- 

 minata are firmly united with the vertebral column. In the 

 Cetaceans, in which the hind-limbs are wanting, and there is 

 no sacrum, the innominate bones are rudimentary, and are not 

 attached in any way to the spine. 



The only other bones which are ever connected with the 

 pelvis are two small bones which are directed upwards from 

 the brim of the pelvic cavity in Marsupials and Monotremes. 

 These are the so-called " marsupial bones," regarded generally 

 as not forming parts of the skeleton properly so called, but 

 as being ossifications of the internal tendons of the " external 

 oblique" muscles of the abdomen (fig. 360). 



In those Mammals which possess hind-limbs, the normal 

 composition of the member is of the following parts : i. A 

 thigh-bone or femur; 2. Two bones forming the shank, and 

 known as the tibia and fibula ; 3. A number of small bones 

 constituting the ankle or tarsus ; 4. The " root " of the foot, 

 made up of the "metatarsus;" 5. The phalanges of the toes 

 (see fig. 243). 



