Posterior Arm & Forearm

 Introduction: Even the novice dissector will do a good job with this one since most of the work today is superficial. What I enjoy about learning the posterior arm and forearm is that many of the muscles and tendons we look at today can be identified in the living if you have your orientation. We will also begin to explore the dorsum of the hand to day as we follow the tendons on their way to the wrist and digits.  Radial Nerve: (C5-T1) One thing you may or may not have noticed about the divisions of the BP is that the anterior divisions contribute to nerves that innervate the anterior compartments while the posterior divisions contribute to nerves that innervate the posterior compartments. Pretty cool huh? Anyway, the radial nerve is the primary nerve of the posterior compartment of the arm & forearm that eventually divides into a deep (motor) and superficial (cutaneous) branch after it passes distal to the elbow. Today we will cut through the lateral head of the triceps to get a better view of the radial nerve, the deep brachial artery and the medial head of the triceps brachii. You can do this by following the radial nerve proximally from where you will find it between brachialis and brachioradialis; or you can follow the nerve distally from where you found it in the triangular interval.  Deep Brachial Artery aka Profunda Brachii: 1st branch of brachial artery, it will be seen next to the radial nerve in the spiral groove of the humerus. Both the Gilroy atlas and the Netters atlas describe the profunda brachii artery as changing its name to radial collateral artery after it passes posterior to the humerus.  Lateral & Medial heads of Triceps: Both of these attach to the humerus and the ulna while only the long head attaches to the scapula. The superior attachments of the medial and lateral heads are separated by the radial groove on the humerus. To insure that you know these attachment points, it is worth the three seconds it takes to find them on the painted skeleton.  Thenar Outcrop Muscles: (thenar from the 17th century Greek meaning the part of the hand with which one strikes) Now that you know that, I will tell you that

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you will not find these muscles on the palm, but in the posterior compartment of the forearm. Listed below are the muscles included in this group, each named according to size and function. You will find them on the dorsal surface of the forearm. o Abductor Pollicis Longus o Extensor Pollicis Brevis o Extensor Pollicis Longus: This muscle tendon is found easily by maximally extending the thumb. As it passes over the dorsal surface of the radius, note a bump that this tendon uses as a pulley, this is called the dorsal tubercle of the radius (aka Lister’s tubercle) Superficial branch of Radial Nerve: This nerve travels along the deep surface of brachioradialis until you see it emerge to provide sensation to the dorsum of the hand in the area of the first 3 metacarpals. Please look at a peripheral nerve distribution illustration for more clarification. You will also notice that it passes superficial to the anatomical snuffbox. Anatomical what? See the next bullet… Anatomical Snuffbox: Tendons of the thenar outcrop muscles make this feature. Inside the snuffbox you can palpate the scaphoid as you move your hand from a position of radial deviation to ulnar deviation. You may also be able to feel a radial pulse as the radial artery passes through this landmark. Next, leave your hand in a dependant position and wait for the veins of the upper extremity to fill and you will notice that the cephalic vein passes across the surface of the snuffbox. Again, take note that the superficial branch of the radial nerve also passes superficial to the snuffbox. Extensor retinaculum: A band of tight connective tissue running in the transverse plane securing the extensor tendons in 6 different compartments on the dorsum of the distal radius and ulna. (p.807 Moore & Dalley 5th Ed.) Brachioradialis: Anatomically this is considered a fusiform muscle so it is designed more for a large range of motion than for explosive power. Just try to convince the full beer in your hand of that fact after this next practical. Extensor Carpi Radialis Longus: Innervated by the radial nerve, please note its attachments to the lateral supracondylar ridge proximally and the base of metacarpal 2 distally. Along with extensor carpi ulnaris, Moore & Dalley describe these muscles as being “indispensable when clenching the fist” Why do you think that is if neither muscle has attachment to the phalanges? Extensor Carpi Radialis Brevis: In close approximation with ECRL, this muscle is the culprit involved with tennis elbow. Incidentally, a proper backhand involves external rotation of the humerus, not a snap of the wrist from flexion to extension, at least this is what Serena Williams was shouting at me as we had lunch yesterday.

 First Dorsal Interosseous muscle: Located between the 1st and second metacarpal, this is the muscle into which the radial artery dives to supply the deep palmar arch. Want a demonstration of this muscle? Place your open hand on the table

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with the ulnar border down and the thumb side up. Now abduct the index finger (digit 2). You can see and feel that muscle working between your first and second metacarpals. Incidentally, this is one way a neurologist can test the integrity of the ulnar nerve’s function in the hand. Extensor Digitorum Communis & Extensor Digiti Minimi: This is the extensor of the fingers (digits 2-5) at the metacarpophalangeal joints. To fully extend these digits, EDC requires the assistance of the lumbricales. (lumbricales are intrinsic hand muscles) The extensor digiti minimi is somewhat of an accessory slip of this muscle that exclusively serves digit 5. Extensor Carpi Ulnaris: Innervation of ECU is the posterior interosseous nerve and its distal insertion is similar to that of FCU, that being, the base of the 5th metacarpal. When viewed from the posterior aspect of the forearm, you can see that the extensor carpi ulnaris and the flexor carpi ulnaris sit on either side of the ulna, thus making part of the ulna only skin deep from elbow to wrist. When we get to the leg in a few weeks you will see that part of the tibia is also just skin deep from knee to ankle. Why is that so interesting? Well both are the medial bones of those limb segments. Similarly, the radius and the fibula are only superficial at their most proximal and distal ends. For the developmental biologists reading these notes, yes, I know that the lower limbs rotate so that the tibia and fibula are in a different orientation (medial / lateral) than earlier in our lives. However, the comparisons made above are still, at least to me, interesting. Common Extensor Tendon (on lateral epicondyle of humerus): While ECRB is the muscle precipitating tennis elbow, it is the common extensor tendon that suffers the microtrauma causing the symptoms of tennis elbow. On a side note, if you look up the definition of Tennis Elbow in the 19th edition of the American Illustrated Medical Dictionary (1942) you get… “Lameness of the elbow due to a strain incurred in playing lawn-tennis.” Imagine your doctor giving you that diagnosis. Anconeus: Considered by early anatomists to be a 4th head of the triceps, the term anconeus is from the greek ankon meaning bend / elbow. You will find this sexy muscle on the lateral forearm attaching to the lateral epicondyle of the humerus and the olecranon process of the ulna. A dense layer of connective tissue lies on its superficial surface and needs to be removed for a proper viewing of this muscle. Or as my mother would say of this tissue, “you make a better door than a window”. Posterior Interosseous Nerve & Artery: You will find these emerging from deep to the belly of the supinator muscle. To see these structures try (gently) moving the extensor digitorum communis laterally or medially until you have a clear view of the supinator muscle. They will be small and surrounded by a thin layer of fat so you will have to clean them off to make them apparent. See the note at the end about things you want to keep straight in your head. Supinator Muscle: Although the biceps is the stronger of the muscles that supinate the forearm, this is the muscle that gets the title supinator. Kind of like when you have a tablemate who shows up for the dissection, really doesn’t do anything, then brags at the bar about how great they are at dissection to the hot

bartender. Thus if your classmates refer to you as “the supinator” this is not a compliment. Innervation is the deep branch of the radial nerve.  Extensor Indicis: This muscle affords us more control over this digit that we use so often with the thumb for precise hand movements. Think of your own example of a precise hand movement involving these digits. Innervation – posterior interosseus nerve. Some things you want to keep straight in your head… • Posterior Interosseous nerve is a branch of the radial nerve (yes also called the deep branch of radial if you are not being very concise) • Anterior Interosseous nerve is a branch of the median nerve (Yes, again, you can still call it median nerve, but I will think you are being intellectually lazy) • Posterior Interosseous & Anterior Interosseous arteries are both from the common interosseous artery. • You of course remember from the previous dissection that the common interosseous artery is a branch of the ulnar artery. -T.S. And for your amusement… This is from the Facts on File Encyclopedia of Word and Phrase Origins 4th Edition by Robert Hendrikson. The word Elbonic was used by the Elizabethans to describe “a rude, awkward verse or sentence that seemed to be pushing with the elbow”. If however you search this word on Google you get the definition “the actions of two people maneuvering for one armrest in a movie theatre.” So next time you are at the movies and you find yourself in competition for the coveted armrest with a stranger, counter his elbonic (modern definition) with some Elizabethan elbonics. As a bonus, using iambic pentameter will either impress or confuse the other person causing them to switch seats out of deference to your intelligence or fear of your mental illness respectively.

And for your edification… One common question I get from students during this lab is “What is snuff and / or what is a snuffbox?” and these notes seem like the appropriate forum to address this inquiry. Snuff is a finely ground dry tobacco that is inhaled through the nose. Why might

someone do something like that you ask? Well, it turns out snuff has a rich history and I am about to give you the highlights. Tobacco is a plant native to the Americas and has been used in various applications by the native Indians centuries before Columbus arrived on its shores. I don’t want to go on too many tangents, in this set of notes, but some of the ways the Indians used tobacco include smoking it, chewing it, sniffing it in the form of a powder, and as an enema. If you need to take a minute and look up what an enema is, please do. Anyways, the explorers of this “new world” returned to Europe with this new plant and snuff became immediately popular among the Spanish and French aristocracy. From that time until the twentieth century, tobacco in a variety of forms was treated as a panacea, used in the treatment for colds, fevers, hysteria, laryngeal spasm, gout, ulcers, ringworm, nasal polyps, and poisonous reptile bites. Similar to the Indians, tobacco was used as an enema in the early 19th century in Europe for such problems as strychnine poisoning, constipation, strangulated hernia, tetanus, hydrophobia, and worms. One European snuffer of some renown is Napoleon, who is said to have sniffed up to seven pounds of snuff per month. Wow, that reminds me of that uncle who would smoke two packs of cigarettes a day. Not everyone was a fan of snuff however; Pope Urban VIII had threatened excommunication to those who used snuff since he believed that sneezing (which as you can imagine would be the first symptom of snuff use) was too close to sexual ecstasy. Only a man who is not having sex would compare a sneeze to an orgasm. In fact the only thing that those two activities have in common is that they are both reflex responses to stimuli that are difficult to suppress. Snuff lost some of its popularity to cigarettes only after the invention of a machine that rolled cigarettes. Prior to that time rolling cigarettes by hand was slow and laborious. Four years after that, the invention of a machine that produced matches didn’t help either as these two combined inventions made cigarette smoking simple and convenient. Currently snuff is an esoteric habit practiced by old German men, the Swedish (approximately 20% of Swedish people use snuff), and annoying hipsters. The anatomical snuffbox as you have certainly noticed by now, creates a good surface for holding a small amount of finely ground tobacco and since people used it that for that purpose, that is how it got its name. References: Sapundzhiev, N., & Werner, J. A. (2003). Nasal snuff: Historical review and health related aspects. The Journal of Laryngology and Otology, 117(9), 686-91. Retrieved from http://search.proquest.com/docview/274947948?accountid=38507 Charlton A. Medicinal uses of tobacco in history. Journal Of The Royal Society Of Medicine [serial online]. June 2004;97(6):292.