VAN 121: Veterinary Gross Anatomy-II (2+2)

FOREARM (DORSOLATERAL ASPECT)

Dissection

• Make two circumferential incisions on the skin, one round the elbow joint and the other round the carpal joint.
• Connect these two by another vertical incision corresponding to the dorsal median line of the forearm.
• Reflect the two flaps of the skin thus formed to the sides. The medial flap should not be reflected beyond the shaft of radius.
• The lateral flap is reflected up to the lateral border of the forearm thus exposing all the extensors of the carpus and digits.
• The dissector in reflecting the skin as the cutaneous nerves should exercise great care and patience and blood vessels passing in the superficial fascia should be left intact for examination.
  • Cephalic vein
    • This is a large vein on the medial face of the forearm.
    • It is the upward continuation of the dorsal metacarpal vein.
    • It passes up the forearm, gains its dorsal aspect and reaches the middle of the arm where it given an anastomotic branch to the median vein and then passes up between brachiocephalicus and superficial pectoral to join the external juglar vein at the thoracic inlet.
  • Cutaneous nerves:
    • There are three cutaneus nerves supplying the skin of the forearm on its lateral, dorsal and medial aspects.
  • Lateral cutaneous nerve of the forearm (Radial)
    •  This is a branch from the radial nerve, which passes under the lateral head of triceps and descends on the lateral aspect of the forearm.
  • Dorsal cutaneous nerve of the forearm (Axillary)
    • This is the continuation of the cutaneous branches of the axillary nerve that passes behind the scapular head of the deltoideus, supplies skin on the lateral aspect of the arm and descends on the dorsal aspect of the forearm.
  • Medial cutaneous nerve of the forearm (Median)
    • Musculocutaneous branch of the median nerve supplies the muscular branch to the terminal part of brachialis and continues down as the cutaneous nerve on the medial aspect of forearm.
  • Radial nerve
    • The radial nerve after detaching muscular branches to triceps and the lateral cutaneous nerve of the forearm in the musculospiral groove, descends as a long cutaneous branch of the dorsal aspect of the forearm related to the cephalic vein, both of which lie on the extensor carpi radialis.
    • It then passes down on the dorsal aspect of the metacarpus, which is to be examined later.

Dissection

• The dissector till now has examined only the superficial structures of this region consisting of cutaneous nerves and vessels in the supreficial fascia, to examine the deeper structures.It requires further preparation as detailed below.
• Cut the cephalic vein above the carpus and reflect it up over the arm along with the lateral, dorsal and medial cutaneous nerves on the forearm leaving the long cutaneous branch of the radial nerve intact to be followed in the metacarpal region.
• Dissect off the superficial fascia, which exposes the deep fascia of the forearm, which is a thick glistening aponeurosis that forms sheaths for the muscles. Slit each sheath of the muscles and remove it carefully.
• As the fibrous sheaths enclosing the tendons are cut, feel with fingers the synovia oozing from the synovial sheaths. Note that the synovial sheaths cannot be seen as distinct entities by dissection.
• Extensors of the carpus and digits are arranged from within outwards in the following order.
  • Extensor carpi radialis
  • Medial digital extensor
  • Common digital extensor
  • Lateral digital extensor and
  • Extensor carpi obliques placed deeper to the above muscles.
• Note: The ulnaris lateralis, a flexor of the carpus placed behind the lateral digital extensor, on the lateral aspect of the forearm.
  • Extensor carpi radialis
    • This is the largest of the extensors and lies on the dorsal face of the radius.
    • It has a large fleshy belly, which is succeeded by a strong tendon, which is attached to the metacarpal tuberosity.
    • The tendon is crossed over by the fine tendon of extensor carpi obliquus.
    • Above the carpus, the tendon passes through a synovial sheath.
  • Medial digital extensor
    • It is a small muscle placed lateral to the proceeding, its conical belly is succeeded by a long tendon, which passes thorugh a synovial sheath at the carpus in common with the tendon of the succeeding muscle.
    • It passes down the metacarpus and reaches the medial digit.
  • Common digital extensor
    • It is a long slender muscle situated lateral to the preceding.
    • It has two heads of origin, one from the humerus and the other from the ulna, which is deeply placed.
    • Its tendon passes through the common synovial sheath as noted above and passes down the metacarpus.
  • Lateral digital extensor
    • It is a strong muscle placed lateral to the preceding and attached in the shaft of radius and ulna.
    • Its tendon passes through a synovial sheath on the lateral aspect of the carpus and passes down the metacarpus.
  • Extensor carpi obliquus
    • It is a small muscle placed deeper to the preceding confined to the lower lateral half on the dorsal face of the radius.
    • By cleaning the fascia between the tendons of the preceding muscles the extent of this muscle can be seen.
    • Its fine tendon is directed inwards, crosses over the tendon of extensor carpi radialis, passes through a synovial sheath on the medial aspect of the carpus and is inserted to the large metacarpal.

Dissection

• Cut the tendons of extensors except that of the obliquus.
• Gently reflect each muscle upwards towards its origin commencing from the extensor carpi radialis and note the nerve and blood supply.
  • Collateral radial artery and radial nerve
    • The artery is the last collateral branch of the brachial artery and the nerve is a branch of the radial nerve.
    • They pass obliquely from within outward under the origins of the extensors supplying them and finally terminate in the ulnaris lateralis.
    • They also supply extensor carpi obliquus. Some times a branch from the common interosseus artery may also supply this muscle.