Part 4: Distal Thoracic Limb

As we move on to our dissection of the antebrachium, note that there is a dense ‘sleeve’ of antebrachial (deep) fascia covering the muscles of the antebrachium.

• Make an incision through the deep antebrachial fascia on the caudal aspect of the antebrachium, from the olecranon (caudal ‘elbow’ region) to the accessory carpal bone (caudal aspect of the carpus).

• Carefully reflect and remove the deep antebrachial fascia.

Identify the extensor muscle group of the antebrachium, which is made up of the muscles on the craniolateral aspect of the antebrachium. This group will include extensors of the carpus and extensors of the digits. The main difference to note between these is that the tendons of the extensors of the carpus will cross the carpus and then stop, while the tendons of the extensors of the digits will continue past the carpus down to the digits. (See DG Fig. 2-20)

• Comment: You may see a thin slip of muscle, called the brachioradialis m., adjacent to the main extensor muscles of the antebrachium. This muscle is not significant for our dissection purposes, and is often removed with the skin; however, you may observe the brachioradialis m. on demonstration animals (it will be more obvious in the cat vs. the dog). (See DG Fig. 2-20)

• Before you trace extensor tendons distally, move to the region of the carpus and identify the extensor retinaculum (band of fascia) extending across the dorsal aspect of the carpal region and attempt to define its proximal and distal margins. This retinaculum binds down the extensor tendons that cross the dorsal aspect of the carpus and helps keep them in place.

Identify extensors of the carpus and trace their tendons of insertion:

• The most cranial muscle in the craniolateral group is an extensor of the carpus (the extensor carpi radialis m.). The tendon(s) of this muscle insert on the dorsal surface of the metacarpal bones, thus it acts to extend the carpus.

• The most lateral muscle in this group is also named as an extensor of the carpus (the extensor carpi ulnaris m., aka ulnaris lateralis m.). The tendon(s) of this muscle insert on the fifth metacarpal bone and accessory carpal bones.

  • Comment: This muscle is grouped with the extensors but plays more of a supportive role of the carpus when the carpus is extended to support weight.

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

• Between the two extensors of the carpus, locate the extensors of the digits. Trace the tendons of these extensors into the manus to see them splitting off to the digits (toes).

• The larger of these is the common digital extensor; its tendon will split into four parts after crossing the carpus, one part extending to the distal phalanx of each of the four main digits.

• The smaller muscle on the lateral aspect is the lateral digital extensor which has a tendon that splits into three parts to extend to digits II, IV, and V.

• Dissection Note: You DO NOT need to dissect the tendons of insertion all the way down to the distal phalanges. Just trace the tendons far enough into the manus to appreciate that they are splitting to go to the digits (rather than stopping at the carpus as the extensors of the carpus would).

Identify the flexor muscle group of the antebrachium, which is made up of the muscles on the caudomedial aspect of the antebrachium. This group will include flexors of the carpus and flexors of the digits. Again, the main difference to note between these is that the tendons of the flexors of the carpus will cross the carpus and then stop, while the tendons of the flexors of the digits will continue past the carpus down to the digits.(See DG Figs. 2-23 and 2-24)

• Before you trace flexor tendons distally, move to the region of the carpus and identify the flexor retinaculum (thick band of fascia) extending across the palmar aspect of the carpal region (covering the carpal canal). This retinaculum binds down the flexor tendons that pass through the carpal canal. Attempt to define its proximal and distal borders.

Identify flexors of the carpus and trace their tendons of insertion:

• The most cranial muscle in the caudolateral group is a flexor of the carpus (the flexor carpi radialis m.). The tendon(s) of this muscle insert on the palmar surface of the metacarpal bones (II & III), thus it acts to flex the carpus. (See DG Fig. 2-23)

• The most caudal and lateral muscle in this group is another flexor of the carpus (the flexor carpi ulnaris m.) and has two parts to it (ulnar and humeral heads). The tendons of this muscle insert on the accessory carpal bone and thus serve to flex the carpus. (See DG Fig. 2-23)

Identify flexors of the digits and trace their tendons of insertion:

• Between the two flexors of the carpus, locate the flexors of the digits. Trace the tendons of these flexors into the manus to see them splitting off to the digits (toes).

  • Note that tracing these tendons will require cutting through the flexor retinaculum on the palmar aspect of the carpus.

• There are two flexors of the digits. The more superficial muscle of this group is the superficial digital flexor (SDF) m. (which covers the deep digital flexor m.). The tendon of the SDF splits into four parts as it crosses the carpus and each part extends down to the middle phalanx of each of the four main digits. (See DG Fig. 2-23)

• The deep digital flexor m. is deep to the superficial digital flexor m. and has multiple heads or ‘bellies’ to it. It has a thick tendon that passes across the carpus and splits into five parts, each part extending to the distal phalanx of each of the five digits.

• Dissection Note: You DO NOT need to dissect the tendons of insertion all the way down to the phalanges. Just trace the tendons far enough into the manus to appreciate that they are splitting to go to the digits (rather than stopping at the carpus as the flexors of the carpus would).

• While dissecting the digital flexor mm., it will be helpful to also identify the carpal canal. The tendons of the flexors of the digits will travel through the carpal canal on the palmar aspect of the carpus on their way to the digits. Note that the carpal canal is just medial to the accessory carpal bone.

  • If not already completed, transect the flexor retinaculum (covering the carpal canal) and reflect it to open the carpal canal.

  • Use a probe to elevate the tendon of the deep digital flexor m. out of the carpal canal to identify it.

On the medial aspect of the limb, return to the brachial region. Re-identify the brachial a. and continue tracing it distally from the brachium into the antebrachium.

• Note that in the cat, the brachial a. and median n. pass through the supracondylar foramen as they continue distally in the limb. (Note that the dog does not have this foramen.) Be sure to note this difference in the cat vs. the dog.

On demonstration specimens, the ‘interosseous’ artery branches of the brachial artery will be dissected for you to observe. In the dog, note the common interosseous a. arising from the brachial a. in the proximal third of the antebrachium. The common interosseous a. is short and passes to the proximal part of the interosseous space between the radius and ulna. It divides into three branches, the ulnar a., the caudal interosseous a. and the cranial interosseous a. Note that the common trunk (common interosseous a.) is usually absent in the cat, but the individual branches listed are usually present. (See DG Figs. 3-29, A & B, 3-30, A & B, and 3-36A)

• On the demonstration specimens, note the cranial interosseous a. coming off of the common interosseous a. in the dog (or separately, as individual branches, in the cat).

  • The cranial interosseous a. passes through the proximal part of the interosseous space cranially; it supplies muscles of the cranial and lateral antebrachium. Only a very short portion of this artery will be visible. (See DG Figs. 3-29A, 3-30B, and 3-36A)

• On the demonstration specimens, note the caudal interosseous a. that lies between the radius and ulna in the interosseous space. To expose this branch, the pronator quadratus m. between the radius and ulna has been cut and the muscle scraped out of the interosseous space with the blunt end of a scalpel handle. (See DG Figs. 3-29A, 3-30B, and 3-36A)

After giving off the common interosseous a. (dog) or individual arterial branches (e.g., cranial and caudal interosseous aa.) (cat), the brachial artery changes names and continues as the median artery.

Identify the median a. and trace it distally through the antebrachium to the level of the carpus. (See DG Figs. 3-29, A & B, 3-30, A & B, and 3-36A)

• Dissection Note: This continuation (median a.) will be large in the dog, but (very) small in the cat. In the cat it may be very difficult to see/follow distally.

In the cat specimens: Identify the radial a. continuing as the main arterial branch in the middle to distal third of the antebrachium; you will see this artery coursing slightly cranially. (See DG Figs. 3-29A, and 3-30, A & B)

• Dissection Note: The radial a. is large in the cat and very small in the dog. You do not need to dissect this artery in the dog specimens.

On demonstration specimens, note the superficial palmar arch given off of the median a.

• Comment: After passing through the carpal canal, the median a. forms the superficial palmar arch with a branch of the caudal interosseous a. This arch then gives rise to the palmar common digital arteries which supply the palmar surface of the forepaw. (See DG Fig. 3-38)

• Note that this arch may be very small and difficult to see in some specimens (especially in cats).

On the medial aspect of the limb, return to the brachial region. Re-identify the median n. and trace its continuation distally into the antebrachium. The median n. will follow the brachial a. into the antebrachium where it continues alongside the median a. The median n. will continue through the carpal canal into the forepaw. The median nerve innervates several muscles of the antebrachium (notably flexor muscles of the carpus and digits) and the skin of the palmar surface of the paw. (DG Figs. 3-30A, 3-32, and 3-38)

• Dissection Note: Transect/reflect muscles as needed to follow the course of the median nerve

  Related Learning Objective

  • D1.7

   

On the medial aspect of the limb, return to the brachial region. Re-identify the ulnar n. and trace its continuation distally into the antebrachium. The ulnar n. courses through the caudal antebrachium, and will divide into a dorsal branch and a palmar branch in the approximate middle of the antebrachium. (These branches need not be dissected.) The ulnar nerve innervates several muscles of the antebrachium (notably flexor muscles of the carpus and digits) and the skin of the caudal aspect of the antebrachium as well as the palmar surface of the paw.(See DG Fig. 3-31)

• Dissection Note: Reflect muscles as needed to follow the course of the ulnar nerve.

On demonstration animals, identify the muscles and other structures described below that are associated with the manus. (See DG Fig. 2-26) (Please note that all terms in this section shown in regular bold font are testable material on quizzes and exams – even though you are not dissecting them on your own specimens!)