The Truncobrachial Junction
Hello, welcome back to another revision post, which is thankfully the last one on anything to do with the forelimb (are you as sick of it as I am yet?!) We have a 'steeplechase' exam in just under two weeks so the next few posts you'll be seeing are embryology and some stuff on ligaments and bone, because I'm definitely confused! :)
Anyway, you may have noticed that on an animal, there's no bony connection to the body on their forelimb. Whereas we have our clavical connecting our shoulder to our body, the dog doesn't have anything, which actually is a very clever adaptation to make sure they run faster (more about that later!) There's no specific name for this area which is a pain, so Cambridge have made up their own name for it - the truncobrachial junction. Trunco- becaue it connects to the...well...trunk, and brachial because it connects the arm to the trunk. It's a synsarcosis (meaning there's no bony attachment) and there's basically a million (exaggeration but whatevsm tis my blog) muscles that go with it.
I've spent an incredibly long time memorising them, so I'm bloody hoping it's paid off otherwise what am I at vet school for? Here we go....
The superficial muscles
You may recall from a few posts ago that we can divide the skin into two layers: superficial (closer to the surface) and deep (more medial, further inwards). Well because of this we have two 'layers of muscles' - the superficial muscles and the deep muscles. We also (very helpfully, not) have these subdivided into 'extrinsic' and 'intrinsic' muscles. Extrinsic muscles describe muscles orginating from outside the limb and inserting in the limb, whilst intrinsict muscles describe muscles orginating and inserting within the limb.
That might all sound confusing - trust me, it took me like a week to actually understand it - but it's not really important for understanding this post. Just as a point of note though, I'll be referring to muscle 'origin' and 'insertion' using the letters 'o' and 'i'. I'm still trying to learn these (there's so many, and then we have to learn which nerves innervate them as well which just sounds...hard. Why did I choose to be a vet again?)
We're going to start off easy - the cutaneous trunci. This is a big muscle on the far right on the trunk, whose primary function is - get this - to twitch the skin in response to irritation! You can actually see it at work in horses removing flies because it's so big in them. It's an extrinsic muscle as it orginates outside the limb.
o. subcutaneous fascia
i. the skin of the trunk.
Moving on, we have cleidobrachialis, cleidocervicalis, and cleidomastoideus (somebody asked me how to remember these the other day and I had to admit it was only because it sounds like 'mastitis' and I think it's a funny word. No particular reason, I just don't see why it's necessary for a word to contain 'tit' unironically). These are the main forelimb protractors, and are also sometimes described together as the 'brachiocephalictus'. They interconnect at the clavicular tendon, which is sort of the remnant of the dog's clavicle. They are all extrinsic muscles.
cleidobrachialis - o. clavicular tendon, i. cranially on humerus shaft distally
cleidocervicalis - o. median raphe of neck, i. clavicular tendon
cleidomastoideus - o. mastoid surface of exoccipital bone of skull (don't worry about what
that means - we don't have to, so neither do you.) i. clavicular tendon
The omotransversarius, cervical trapezius, and thoracic trapezius are the caudal (towards the bum) rotators of the scapula. Remember we talked about the dog adapting to run faster? Basically, the scapula acts as the fulcrum as the limb (the point where it rotates) when the dog runs. If it was attached to a clavicle, the limb would have to rotate at the shoulder, which is lower down. If you think about it, this would mean that the dog's stride length was decreased, so it wouldn't be able to run as fast. (Yes, it sounds a bit physicsy so I did fall asleep until I realised it was something I should probably learn). These are all extrinsic as well.
omotransversarius - o. wing of atlas, i. distal scapular spine
cervical trapezius - o. caudal half of distal head raphe, i. proximal scapular spine
thoracic trapezius - o. dorsal median of thorax raphe, i. proximal scapular spine
(I've just thought, you may be wondering what raphe actually is. It actually has an accent on the e at the end, I'm just too lazy to figure out how to type that easily so I've left it on, and I don't actually know. Google, however, reckons its where two halves of an embryo fused producing a seam/groove. Makes sense to be fair.)
The last few muscles we'll be talking about at the superficial layer are the deltodei. The acromiodelotid and the spinodeltoid are both intrinsic muscles, and whilst technically the lateral and long head of triceps are superficial muscles, I've grouped them into the deep muscles below for reasons you'll see in a moment.
These muscles flex the shoulder on the lateral (towards the outside) side, whereas the teres major (discussed below) is the medial flexor.
spinodeltoid - o. scapular spine, i. deltoid tuberosity on humerus
acromiodeltoid - o. acromion, deltoid tuberosity on humerus
Whilst I haven't really talked about the positioning of these muscles, I have talked about their function and whether they originate inside or outside the limbs. In theory, this should be enough for you to figure out their location in the diagram below, but to be fair it's mostly for my own revision to design them, so no big deal if you can't!
The deep (profundus) muscles
Most muscles at this layer are intrinsic muscles (originate from within the limb). There is one exception to this rule however, and that's the latissimus dorsi, which is the rather large muscle you can see on the right of the diagram below. It's a prime retractor of the forelimb, and is an extrinsic muscle.
o. spinous processes of trunk vertebrae (L1-L7)
i. teres tuberosity of the humerus
We also have the cervical trapezius which is an extrinsic muscle, along with the thoracic trapezius. These aren't amazingly special muscles as they're really suspected to just elevate the scapula, but they're still important to know.
o. dorsal median raphe of neck (C2-L3) (cervical trapezius) and dorsal median raphe of thorax (thoracic trapezius), i. both on cranial angle and vertebral border of scapula.
The cervical serratus ventralis is believed to be the cranial (towards the head) rotator of the scapula (see above for the caudal rotators), and it opposes the trapezeii's actions.
o. neck vertebra
i. serrated area of scapula (cranial end)
The thoracic serratus ventralis merges imperceptibly with its cervical partner, so I haven't included it on my diagram because I have no idea where I would put it! However it is the prime depressor of the limb.
o. ribs 1-8
i. serrated area of scapula (caudal end)
The deep pectoral muscles prevent the limb being splayed out of the sagittal plane of the body - they are adductors, and are vital in cursorial species (species adapted for running).
i. lesser tuberosity, tendinous slips to greater tuberosity
Coming back to the teres major, this is said to be the medial flexor of the shoulder joint. It looks like a strap and helps to balance the shoulder so that it does not adduct (get splayed outwards). Intrinsic mucle.
o. caudal border of scapula
i. teres tuberoisty
Similarly, there is one prime extensor of the limb and it is also an intrinsic muscle, known as the supraspinatus. You would have to look on the medial side of the scapula to see this, along with the infraspinatus which is discussed later.
o. supraspinous fossa
i. greater tuberosity
There are some muscles which are designed to stabilise the shoulder joint, since it doesn't have ligaments in the same way that a joint such as the elbow does. These are know as 'ligamentar' muscles (I know, original right?) and are the infraspinatus (lat) and the subscapularis (med).
Infraspinatus - o. infraspinous fossa, i.lateral side of greater tuberosity
subscapularis - o. subscapular fossa (all the space distal to the serrated area on the medial scapula), i. lesser tuberosity.
You may at this point be thinking surely she's done? It feels like I've spent an hour prattling on about muscles (I actually have) and surely must be done by now? Nope! Folks, you've got another three to go - invigorating right? If you're alreadt falling asleep and would just like to read my sarcastic conclusion, I won't be offended if you skip straight to the end. It's exactly what I would be doing at this point.
The tensor fasciae antebrachii is an elbow extensor and is a bit boring so I can't be bothered to say much else about it.
o. lateral side of latissimus dorsi
i . olecranon and antebrachial fascia (where the hell is this????)
If you weren't bored already, get ready for the triceps. Annoyingly, it's actually got four heads in the dog, so triceps is really a bit of a shit name for it. Quadraceps? Fixed it. Anyway, the four heads of the triceps (eye roll) are the accessory head, the lateral head, the long head, and the intermediate head. All four of these insert on the olecranon of the elbow and if you're really being a pain you'll make me remember where they all originate as well (basically, all of them except the long head come from somewhere on the humerus, whereas the long head comes from the caudal border of the scapula).
Are we done? Of course not!
Our final friend is the biceps which, much like our friend the triceps has also decided to be a little shit in the dog because it just has ONE head, not two as any sensible person would suspect it would have.
o. bicipital tuberosity on scapula
i. ulnar tuberosity
Oh wait no, I forgot one. Brachialis! This is the other elbow flexor and spirals around the musculospiral groove from its origin (caudo-laterally on humerus) to its insertion on the ulnar tuberosity.
If you aren't asleep yet, there's a diagram for you to test yourself on and feel woefully inadequate.
If you've made it this far, congratulations. I definitely wouldn't have, and I'm voluntarily studying this!
I think anatomy is definitely my favourite part of the degree so far, but jesus it can be dry. On the bright side, my new party trick is that I can put people to sleep in under 10 seconds by using this brand new method. If you really want to make sure people don't invite you to parties, you should try it sometime!