The forelimb is where most students first meet the size of veterinary anatomy: dozens of muscles, each with an origin, an insertion, an action and a nerve. Learned as a list, it falls apart a week later. Learned by nerve, it holds, because the nerve tells you the muscle's job, and an injury to the nerve tells you what the dog will look like.
Where the nerves come from
The forelimb is supplied by the plexus brachialis, formed in the dog mainly from the ventral branches of the sixth, seventh and eighth cervical and the first thoracic spinal nerves, with small contributions from the fifth cervical and second thoracic in some dogs.1 Learn those four numbers, C6 to T1, first; the rest hangs on them.
The seven families
| Nerve | Main roots | What it supplies | So it |
|---|---|---|---|
| N. suprascapularis | C6, C7 | Supraspinatus, infraspinatus | extends and stabilises the shoulder |
| Nn. subscapulares | C6, C7 | Subscapularis | holds the shoulder medially |
| N. axillaris | C7, C8 | Deltoideus, teres major and minor | flexes the shoulder |
| N. musculocutaneus | C6 to C8 | Biceps brachii, brachialis, coracobrachialis | flexes the elbow |
| N. radialis | C7 to T1 | Triceps brachii, anconeus, the extensors of the carpus and digits | extends the elbow, carpus and digits |
| N. medianus | C8, T1 | Pronators, flexor carpi radialis, digital flexors in part | flexes the carpus and digits |
| N. ulnaris | C8, T1 | Flexor carpi ulnaris, digital flexors in part | flexes the carpus and digits |
The rule of thumb for the forearm follows from the table: the extensors on its cranial side are radial; the flexors on its caudal side are median and ulnar.
Three that leave the limb
Three branches of the plexus go to the trunk and are easy to forget until an exam asks. The n. thoracodorsalis supplies the latissimus dorsi, the n. thoracicus longus the serratus ventralis, and the n. thoracicus lateralis the cutaneus trunci, the muscle that twitches the skin in the panniculus reflex.1
Why the grouping pays: two dogs
A dog with the radial nerve injured high in the arm cannot extend the elbow, so the limb folds and will not bear weight. Injured lower down, past the branches to the triceps, the elbow holds but the carpus and digits cannot be extended, and the dog knuckles over onto the back of the paw. Knowing that the radial nerve owns the extensors lets you predict both.
A dog with a brachial plexus avulsion after a road accident may lose the panniculus reflex on the same side, because the lateral thoracic nerve comes from the caudal roots, and may show Horner's syndrome when the first thoracic roots are torn, because the sympathetic supply to the eye leaves the cord there.1
Doing it in the atlas
- Open the dog and hide the skin layer, then turn to the left view with the 1 key.
- Select a muscle, the triceps for instance, and read its innervation in the reading pane. Each written fact names the reference it is quoted from.
- Work through the table one nerve at a time, selecting each muscle and checking that the pane agrees with you.
- Use Isolate to see one family alone, then Fade others to put it back in context.
- Finish with a quiz by body region on the thoracic limb, and review the mistakes.
The table is the textbook account. If the atlas and the table ever disagree for a muscle, the reading pane tells you its source; tell us too, and we will look at it.
Sources
- Evans HE, de Lahunta A. Miller's Anatomy of the Dog, 4th edition. Elsevier, 2013
- Nomina Anatomica Veterinaria, 6th edition. International Committee on Veterinary Gross Anatomical Nomenclature, 2017