The terminology applied to the synsacral vertebrae by different authors varies. The terminology proposed by DuToit (1913) is employed in the present account. See my [Fig. 1]B. This terminology differs considerably from that used by Howard (1929). DuToit divides the fused synsacral vertebrae into the following five groups, listed in anteroposterior sequence: (1) synsacro-thoracic, which bear movable ribs; (2) synsacro-thoraco-lumbar, which lack movable ribs but possess well developed laterally directed parapophyses, in addition to the more dorsally directed diapophyses; (3) synsacro-lumbar, which lack parapophyses, although possessing inconspicuous diapophyses; these vertebrae are shortened anteroposteriorly and are so firmly fused together that often the number present can be determined only by counting the intervertebral foramina; (4) synsacro-sacral, which have much more pronounced transverse processes than do the synsacro-lumbar vertebrae; these transverse processes are expanded distally where they fuse with the ilium and represent both parapophyses and diapophyses partly or completely fused together plus sacral ribs (detectable only in the embryo); there are considered to be two of these vertebrae; they are situated at approximately the level of the acetabulum; (5) synsacro-caudal, which include the remainder of the fused vertebrae; no marked gross morphological features differentiate the synsacro-sacral and the synsacro-caudal groups of vertebrae. The boundaries between all but the last two groups of vertebrae are usually, but not always, easily determined. It may be difficult to determine whether a vertebra with rudimentary parapophyses belongs to the synsacro-thoraco-lumbar or the synsacro-lumbar group. Sometimes a parapophysis will be better developed on one side of a vertebra than on the other.
Femur
The trochanter is a large squarish tuberosity on the lateral surface of the proximal end of the femur. The trochanteric ridge is a sharp, longitudinal (relative to the femur) ridge forming the anterior edge of the trochanter. The obturator ridge is a short, blunt, longitudinal ridge forming the posterior edge of the trochanter. The anterior intermuscular line is a slight ridge extending distally from the trochanteric ridge. The posterolateral intermuscular line is a slight ridge extending distally from the obturator ridge. The posterior intermuscular line is a slight, longitudinal ridge on the mid-posterior surface of the femur. The internal condyle is a large rounded articular prominence on the medial side of the distal end of the femur. On the lateral side of the distal end of the femur are two articular prominences—the lateralmost, smaller one is the fibular condyle, separated by the fibular groove (visible from posterior aspect only) from the larger and more medial external condyle. The popliteal area is a depression on the posterior surface of the distal part of the femur immediately proximal to the condyles.
Tibiotarsus and Fibula
The inner cnemial crest is pronounced and directed anteriorly on the anterior surface of the proximal end of the tibiotarsus. The outer cnemial crest is pronounced and directed anterolaterally on the anterolateral surface of the proximal end of the tibiotarsus. The rotular crest is transverse and forms the anterior border of the proximal end of the tibiotarsus; the crest extends between the dorsal ends of the two cnemial crests and also extends medial to the inner cnemial crest. The fibular crest is longitudinal on the lateral surface of the tibiotarsus and fuses with the middle part of the fibula. The fibular tubercle is small and on the lateral surface of the fibula near the level of the middle of the fibular crest. The anteromedial intermuscular line is a slight ridge extending from the inner cnemial crest down the anteromedial surface of the tibiotarsus. The anterolateral intermuscular line is a slight ridge extending from the fibular crest down the anterolateral surface of the tibiotarsus. The supratendinal bridge is a transverse bony arch over a longitudinal groove near the distal end of the anterior surface of the tibiotarsus.
Tarsometatarsus
The hypotarsus is a large, pronounced, squarish protuberance on the posterior surface of the proximal end of the tarsometatarsus and contains grooves and canals for the passage of the flexor tendons. The longitudinal ridges forming the lateral and medial edges of the posterior surface of the hypotarsus are termed calcaneal ridges. The posterior metatarsal crest is long and sharp; it is continuous with the medial calcaneal ridge that extends most of the way down the posterior surface of the tarsometatarsus medial to the midline; there is an opening between this crest and the tarsometatarsus immediately distal to the hypotarsus. The medial metatarsal depression is large; it is on the medial surface of the proximal end of the tarsometatarsus. The anterior metatarsal groove is a longitudinal groove in the midline of the proximal part of the anterior surface of the tarsometatarsus. The three trochleae are large rounded articular prominences at the distal end of the tarsometatarsus; there is one at the base of each of the digits II, III, and IV. The term distal foramen (as used by Howard) refers to a short, anteroposteriorly directed canal that perforates the tarsometatarsus a short distance proximal to the intertrochlear notch between the trochleae for digits III and IV. Beginning at the middle of this canal and extending distally at a right angle to it is the intertrochlear canal, which opens via the terminal foramen into the intertrochlear notch between the trochleae for digits III and IV.
Nerves