Part 3: Distal Pelvic Limb

As we move on to our dissection of the crus, note that there is a dense ‘sleeve’ of deep crural fascia covering the muscles of the leg.

• Make an incision through the deep crural fascia craniomedially throughout the length of the crus.

• Reflect this thick fascia (and, if needed, remove it) to view the craniolateral muscles of the crus. Be careful to avoid damaging the lateral saphenous v. as you reflect/remove the fascia.

On the craniolateral aspect of the crus, identify the muscles that are flexors of the tarsus and extensors of the digits. The main difference to note between these types of muscles is that the tendons of the flexors of the tarsus will cross the tarsus and then stop, while the tendons of the extensors of the digits will continue past the tarsus down to the digits.

• Identify the flexors of the tarsus and trace their tendons of insertion:

  • Included in the flexors of the tarsus, you should specifically identify the cranial tibial m. This is the most cranial muscle in the crus. Trace the tendon of this muscle distally to view its insertion point on the first and second metatarsals.

    • • • As you trace the cranial tibial tendon distally, notice that its tendon runs under an extensor retinaculum (the crural extensor retinaculum) as it nears the distal end of the crus. (See DG Figs. 2-43, 2-53 and 2-54)

  • Moving laterally from the cranial tibial muscle, there is another flexor of the tarsus (the fibularis (peroneus) longus m.). Trace the tendon of this muscle to note its insertion on the fourth tarsal and base of the metatarsal bones. (See DG Figs. 2-43, 2-53 and 2-54)

    • • • Comment: Note that you may see another flexor of the tarsus (the fibularis brevis m.) deep to the fibularis longus in the region of the crus but it is not significant for our dissection purposes. (See DG Figs. 2-43 and 2-54)

• Identify extensors of the digits and trace their tendons of insertion:

  • Just deep to the cranial tibial m. (on the lateral side), you should specifically identify the long digital extensor m. This muscle is sandwiched between the two main flexors of the tarsus. Trace the tendon of this extensor into the pes to see it splitting into four parts and extending to the digits (toes). (See DG Figs. 2-43, 2-53 and 2-54)

    • • • As you trace the long digital extensor tendon distally, notice that it’s tendon runs under two extensor retinacula (first, the crural extensor retinaculum at the distal end of the crus, then the tarsal extensor retinaculum as it crosses the tarsus). (See DG Figs. 2-43, 2-53 and 2-54)

  • On the lateral aspect of the crus, note that there is another small digital extensor m. (the lateral digital extensor m.) Trace the long and thin tendon of this muscle distally as it extends toward the lateral most digits of the pes. (See DG Figs. 2-43 and 2-54)

  • Dissection Note: You DO NOT need to dissect the tendons of insertion of these extensor muscles all the way down to the distal phalanges. Just trace the tendons far enough into the pes to appreciate that they are splitting to go to the digits (rather than stopping just past the tarsus as the flexors of the tarsus would).

    Related Learning Objective

    • D2.4

     

Return to the cranial aspect of the crus and dissect deep to the cranial tibial m. to identify the cranial tibial artery. This artery lies on the deep face of the cranial tibial m., between the cranial tibial m. and the long digital extensor m. The cranial tibial a. supplies the flexors of the tarsus and the long digital extensor m. (See DG Figs. 4-55, 4-61, and 4-72).

On the caudal aspect of the crus, identify the muscles that are extensors of the tarsus and flexors of the digits. The main difference to note between these types of muscles is that the tendons of the extensors of the tarsus will cross the tarsus and then stop, while the tendons of the flexors of the digits will continue past the tarsus down to the digits. 

• Identify extensors of the tarsus and trace their tendons of insertion:

  • Identify the gastrocnemius m. which is made up of two heads, medial and lateral. This muscle makes up the majority of the ‘bulge’ we see on the caudal aspect of the leg (crus) and is a major contributor to the common calcanean tendon. The gastrocnemius is an extensor of the tarsus and also a flexor of the stifle joint. (See DG Figs. 2-42, A & B, 2-43, 2-54 and 2-55, B & C)

    Related Learning Objectives

    • D2.3
    • D2.6

     

    • • • Identify both medial and lateral heads of the gastrocnemius m.

        • Identify the common calcanean tendon. This is the very thick tendon seen extending distally from the gastrocnemius m. to the hock where it inserts on the calcaneus (of the tarsus). Several muscles contribute to the formation of this tendon; the major contributors are the gastrocnemius m. and the superficial digital flexor m., but there are also minor contributions from the biceps femoris, gracilis, and semitendinosus mm. (See DG Figs. 2-42, A, and 2-55, B)

        • Comment: Note that there are fabellae in the tendons of origin of the gastrocnemius m. There is one sesamoid bone (fabella) found in each tendon of origin (i.e., one in the tendon of the medial head and one in the tendon of the lateral head.)

  • In the cat, identify the soleus m. (not present in the dog) which is found just deep to the lateral head of the gastrocnemius m. The soleus m. also contributes to the common calcanean tendon (in the cat).

  • Transect the lateral head of the gastrocnemius near its origin and reflect it off of the underlying digital flexor m. (This is the superficial digital flexor muscle – it is sandwiched between the two heads of the gastrocnemius m.)

    • • • Dissection Note: When making this transection, try to avoid cutting the nerve found crossing the lateral head of the gastrocnemius and the nerve diving between the two heads of the gastrocnemius.

• Before you trace flexor tendons distally, move to the region of the tarsus and identify the flexor retinaculum (thick band of fascia) extending across the medial plantar aspect of the tarsal region (covering the tarsal canal). This retinaculum binds down the flexor tendons that pass through the tarsal canal.

• Identify flexors of the digits (which also serve as extensors of the tarsus) and trace their tendons of insertion:

  • Locate the main flexors of the digits. Trace the tendons of these flexors into the pes to see them splitting off to the digits (toes).

    • • • Note that tracing these tendons will require cutting through the flexor retinaculum on the medial plantar aspect of the tarsus.

        • There are two main flexors of the digits. The more superficial muscle of this group is the superficial digital flexor (SDF) m. (which is deep to, and sandwiched between, the two heads of the gastrocnemius m.). This muscle is a flexor of the digits but also a flexor of the stifle joint and extensor of the tarsus. (See DG Figs. 2-42, A & B, 2-54, and 2-55, B & C)

          Related Learning Objectives

          • D2.3
          • D2.6

           

          • • • Trace the tendon of the superficial digital flexor m. distally over the hock. Be sure to trace the tendon into the metatarsus to see where it splits into four parts and will then extend to the middle phalanx of each of the four main digits.

              • Make a sagittal incision just lateral (or medial) to the tendon of the superficial digital flexor m., where it becomes superficial and passes over the tuber calcanei.

              • Displace the tendon to one side to observe the underlying calcaneal bursa (shiny pocket w/fluid intended to reduce friction) deep to the tendon of the superficial digital flexor m. as it crosses over the tuber calcanei.

        • Deep to the superficial digital flexor m., identify the deep digital flexor m. which has two parts, the lateral digital flexor m. and medial digital flexor m. The lateral digital flexor is the larger of the two muscles (and is found more laterally) while the medial digital flexor m. is smaller (and is found more medially). The deep digital flexor m. is a flexor of the digits and an extensor of the tarsus. Trace the tendons of the medial and lateral digital flexor mm. distally. (See DG Figs. 2-42, A & B, 2-43, 2-54, and 2-55, A, B & C)

          • • • While tracing the digital flexor tendons distally, you will need to transect the flexor retinaculum (thick band of fascia) extending across the plantar aspect of the tarsal region (covering the tarsal canal). This retinaculum binds down the deep digital flexor tendons that pass through the tarsal canal.

              • Use a scalpel to carefully cut through the flexor retinaculum. Continue to trace the tendons of the deep digital flexor mm. as they run under the retinaculum and continue toward the pes.

              • As you dissect the deep digital flexor tendons, elevate them slightly to identify the tarsal canal (the space occupied by the tendons) just medial to the calcaneus.

        • Dissection Note: You DO NOT need to dissect the flexor tendons of insertion all the way down to the phalanges. Just trace the tendons far enough into the pes to appreciate that they are splitting to go to the digits.

        • Observe the popliteus m. proximal to the deep digital flexor m., on the caudal surface of the proximal tibia (which is best seen from the medial aspect). At the tendon/muscle junction, there is a sesamoid bone. The popliteus m. acts to rotate the leg medially. (See DG Fig. 2-55, A & B)

On the lateral side of the limb, return to the thigh region to trace the path of the sciatic nerve distally. Reflect the biceps femoris m. cranially and trace the sciatic n. distally in the limb to identify where it branches into the common fibular (peroneal) n. and the tibial n.

• Identify the common fibular (peroneal) n., which is the smaller, more cranial, terminal branch of the sciatic n. The common fibular n. will pass laterally across the lateral head of the gastrocnemius m. and between the lateral digital flexor and fibularis longus mm. To supply the muscles on the cranial aspect of the crus.

  • Comment: Note that the common fibular n. will subsequently divide into superficial and deep fibular (peroneal) nn. but you need not dissect these branches. These branches innervate the flexors of the tarsus and extensors of the digits. (See DG Figs. 4-66 and 4-70)

• Identify the tibial n., which is the larger, more caudal, terminal branch of the sciatic n. The tibial n. will dive in between the medial and lateral heads of the gastrocnemius m. The tibial nerve supplies the caudal muscles of the crus which include the extensors of the tarsus and flexors of the digits. (See DG Figs. 4-59, 4-61, 4-62, 4-66, 4-68, and 4-70)

  Related Learning Objective

  • D2.4

   

On the cranial aspect of the limb, re-identify the cranial tibial a. and trace its path as it continues distally. As the cranial tibial a. passes over the talocrural joint it becomes the dorsal pedal a. Identify the dorsal pedal a. (See DG Figs. 4-71 and 4-72)