CHARACTERS OF MAMMALIA. 477 



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 

 Qiiadrumana 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. Generally speaking, however, the poste- 

 rior 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 re- 

 main disunited during life. As a rule, also, the ossa innomin- 

 ata are firmly united with the vertebral column. In the Ceta- 

 ceans, 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. 187). 



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. 120). 



The thigh-bone or femur articulates with the pelvis, usually 

 at a very open angle. In Man it is distinguished by being the 

 longest bone of the body, and by having the axis of its shaft 

 nearly parallel to that of the vertebral column. In most 

 Mammals the femur is relatively shorter, and the axis of its 

 shaft deviates considerably from that of the spine, being some- 

 times at right angles, or even at an acute angle. 



