MAMMALS. 



atlas ; following which is the axis vertebra, remarkable for having the body or 

 basal portion of the atlas vertebra fixed to it, and known as the odontoid process. 

 This separation of the body of the atlas vertebra from its proper segment is 

 constant throughout the greater part of the vertebrate subkingdom. The remaining 

 five of the cervical, or neck- vertebrae, are distinguished from the dorsal, or vertebrae 

 of the region of the chest, by the absence of ribs. The ribs of most of the dorsal 

 vertebrae articulate in the middle line of the inferior aspect of the body with the 

 breast-bone, or sternum, which is itself composed of several segments. The dorsal 

 vertebrae are succeeded posteriorly by a smaller number, forming the region of the 

 loins, which have no ribs, and are termed lumbars. Behind the latter there are 

 several coalesced vertebrae forming the so-called sacrum, to which the haunch-bones 

 articulate ; and these are again succeeded by the tail, or caudal, vertebrae, of which 

 the number varies according to the length of the tail itself. 



In the majority of Mammals the fore-limb is 

 connected with the trunk simply by the blade-bone, 

 or scapula, which lies on the back surface of the 

 anterior ribs; and in front by the collar-bone, or 

 clavicle, which connects the scapula with the ster- 

 num. The bones of the fore-limb are, firstly, the 

 arm-bone or humerus, which has condyles at its lower 

 end ; and, secondly, the two parallel bones of the 

 fore-arm, of which the outermost (when the palm of 

 the hand is turned forwards) is the radius, and the 

 other the ulna. The radius is always present, but 

 in many Hoofed Mammals only the upper end of the 

 ulna remains, which is fused with the radius. The 

 radius articulates below with the upper of the two 

 transverse rows of small solid bones forming the 

 wrist or carpus; beyond these we have in man and 

 monkeys, as well as in certain other groups, five 

 elongated bones, termed metacarpals, the four outer- 

 most of which are succeeded by the three phalangeal 

 bones of the fingers or digits. The thumb, or first 

 digit, which lies on the same side as the radius, has, 

 however, only two of these phalangeals. 



The hind-limb differs from the fore-limb in that 

 the innominate, or haunch - bones, which together 

 form the pelvis, are connected by an immovable bony 

 union with the sacral region of the vertebral column. The thigh-bone or femur, 

 corresponding to the humerus of the arm, articulates with a cavity in the innominate, 

 termed the acetabulum. The leg has two parallel bones articulating with the 

 lower end of the thigh-bone or femur ; of which the larger, or tibia, occupying the 

 inner side of the limb, corresponds to the radius of the fore-arm, while the smaller 

 outer bone, or fibula, represents the ulna. The ankle, or tarsus, corresponds to the 

 carpus in the fore-limb, and likewise consists of two transverse rows of small 

 bones. Two bones of the uppermost row, viz. the calcaneum or heel-bone, and 



DIAGRAM OF THE BONES OF THE LEFT 

 FORE-FOOT OF A FIVE-TOED MAM- 

 MAL (Phenacodus). (% nat. size.) 



The bones marked 5, I, c, t, t d , m, 

 and u, form the wrist ; those numbered 

 I-V are the metacarpals, and the re- 

 mainder the finger or toe -bones. 

 After Osborn. 



