THE BONES OF THE PELVIC LIMB 29 



the weight, while the second and fifth are reduced. The existing horse has a single 

 digit, the third of his polydactyl ancestors. The skeleton of each fully developed 

 digit consists of three phalanges and certain sesamoid bones. The first or proximal 

 phalanx (Phalanx prima) articulates with the corresponding metacarpal bone above 

 and with the second or middle phalanx (Phalanx secunda) below. The third 

 or distal phalanx (Phalanx tertia) is inclosed in the hoof or claw, and is modified to 

 conform to the latter. The sesamoid bones (Ossa sesamoidea) are developed 

 along the course of the flexor tendons or in the joint capsules. Two proximal 

 sesamoids (Ossa sesamoidea phalangis primae) occur at the flexor side of the meta- 

 carpo-phalangeal joint and form a pulley for the flexor tendon. The distal 

 sesamoids (Ossa sesamoidea phalangis tertiae) are similarly placed l^etween the 

 deep flexor tendon and the joint between the second and third phalanx; they are 

 absent in the dog, which has a small sesamoid on the extensor side of the meta- 

 carpo-phalangeal joints, and often at the proximal interphalangeal joint also. 



Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism) 

 in the horse and other animals. In some pigs, on the other hand, the two chief digits are fused, 

 and the condition appears to be inherited. 



THE BONES OF THE PELVIC LIMB 



The pelvic limb, like the thoracic, consists of four segments, viz., the pelvic 

 girdle, thigh, leg, and the hind foot or pes; the last is subdivided into tarsus, 

 metatarsus, and digits. 



The pelvic girdle (Cingulum extremitatis pelvina) consists of the os coxae or 

 hip bone, which joins its fellow of the opposite side ventrally at the symphysis 

 pelvis, and articulates very firmly with the sacrum dorsally. The two coxal bones, 

 together with the sacrum and the first two or three coccygeal vertebrae, constitute 

 the bony pelvis. The os coxae consists originally of three flat bones, the iUum, 

 ischium, and pubis, which meet at the acetabulum, a large cotyloid cavity that 

 articulates with the head of the femur. These three parts are fused before growth 

 is complete, but are considered separately for convenience of description. The 

 ilium (Os ilium) is situated in the lateral wall of the pelvis, the pubis (Os pubis) in 

 the anterior part, and the ischium (Os ischii) in the posterior part of the ventral 

 wall. 



The thigh (Femur), like the arm, contains a single large, long bone, the femur 

 or thigh bone (Os femoris). This articulates with the acetabulum above and the 

 tibia and patella below. 



The skeleton of the leg (Crus) comprises three bones (Ossa cruris), viz., the 

 tibia, fibula, and patella. The tibia is a large prismatic long bone which supports 

 the weight, and articulates below with the tibial tarsal bone. The fibula is situated 

 along the outer border of the tibia, from which it is separated by the interosseous 

 space of the leg. It is much more slender than the tibia and does not articulate 

 with the femur. In the pig and dog it has a complete shaft and two extremities, 

 but in the horse and ox it is much reduced and otherwise modified. The patella 

 or "knee-cap" is a short bone which articulates with the trochlea of the distal end 

 of the femur ; it is to be regarded as a large sesamoid bone intercalated in the tendon 

 of the quadriceps femoris muscle. 



The tarsus or "hock" consists of a group of short bones (Ossa tarsi) numbering 

 five to seven in the different animals. The proximal or crural row consists of two 

 bones, the tibial and fibular tarsals ; the former is situated at the inner or tibial 

 side, and has a trochlea for articulation with the distal end of the tibia; the latter, 

 situated externally, has a process, the tuber calcis, which projects upward and 

 backward and constitutes a lever for the muscles which extend the hock joint. The 

 distal or metatarsal row consists of four bones when seven tarsal elements are 



