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 pelvinæ) consists of the =os
coxæ= 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 vertebræ, constitute the bony pelvis. The os coxæ
consists originally of three flat bones, the =ilium=, =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 present, as in the pig and dog. They are best
designated numerically as =first tarsal=, =second tarsal=, etc. The
=central tarsal= is interposed between the rows.
The preceding terms are anglicized abbreviations of those introduced
by Gegenbaur into comparative anatomy. The Latin names and synonyms
are given in the following table.
Tibial (Os tarsi tibiale, Tt.) Astragalus or Talus
Fibular (Os tarsi fibulare, Tf.) Calcaneus or Os calcis
Central (Os tarsi centrale, Tc.) Scaphoid or Navicular
First Tarsal (Os tarsale primum, T1) First or internal cuneiform
Second Tarsal (Os tarsale secundum, T2) Second or middle cuneiform
Third Tarsal (Os tarsale tertium, T3) Third or external cuneiform
Fourth Tarsal (Os tarsale quartum, T4) Cuboid.
The =metatarsal= and =digital bones= resemble in general those of the
corresponding regions of the thoracic limb; the differential features
will be noted in the special descriptions.
[Illustration: