• Bella Rawson

Bones of the Forelimb

Updated: Oct 25, 2020

VAP has been getting busy, and in our first supervision we discussed the bones of the canine forelimb. We touched on the hindlimb as well, but in our main VAP course we'e been concentrating on the forelimb, so I thought I'd do today's blog post as some kind of revision!


The overview of the limb looks very much like this, a picture which I included in this week's weekly vet school diaries:


Scapula

I thought we'd start at the top and work our way down to the bottom, so first up is the scapula! I'm viewing this in my mind as the left forelimb, so the view point is going to be lateral (facing away from the dog). This is the flat object at the top of the leg, with a long line of bone running down the middle of it known as the 'spine of scapula'.


It has three borders: the vertebral border running along the top, the cranial border running cranially (the side of the head) and the caudal border facing caudally (towards the bum!) These also give rise to the cranial angle (the bump at the top) and the caudal angle (the bump on the caudal side).


On either side of the spine of scapula, the two large, flat surfaces are known as the supraspinous fossa (above the spine of scapula) and the infraspinous fossa (below the spine of scapula).


On the cranial border lies the neck, an indipping of this side as it narrows to a tip. Here, you'll see the spine widen into a lump and that's called the acromion. On the cranial side of the acromion, you can just about see what's known as the bicipital tuberosity, and if you imagined flipping the scapula over, you would see a lump of bone known as the coricoid process (although, for many animals this is barely visible and a bit rubbish!)


Finally, also of note on that side is the glenoid cavity on the otherside of the acromion (a big hole!) and the serrated area which is at the top of the medial side.


I've created a labelling game if you want to test yourself to see how much you remembered! Just press each of the pins and select the correc option! I haven't included the medial side of the scapula here, just the lateral.

Humerus

Following on, after the shoulder joint, we have the head of the humerus. In real life this is quite rounded, but in my rubbish diagram it looks a bit flat! To the left, you can see a line converging, and this is what is known as the bicipital groove - in real life, this would be something you could run your fingers over and actually feel.


Then we have what's known as the greater tuberosity (lateral) and lesser tuberosity (medial). These are also lumps and bumps at the top of the humerus. If you've forgotten what a tuberosity is, take a look at the terminology key at the top of this page.


Then we have the shaft. This refers to the rectangular portion of bone in between the two bumpy bits, and if you had this bone in your hand in real life you'd be able to feel what's called the muscolospiral groove. Basically, the bone is twisted to allow for the radial nerve and deep brachial artery.


There are a few more less obvious bumps on the shaft before you reach the bottom, and these include the deltoid tuberosity (lateral) and the teres tuberosity (medial).


Finally, you reach the bottom, and if you were viewing this from the other side you would see the trochlea. The bumps here are much more obvious, and we have the medial condyle and the lateral condyle that we can see in the diagram. Behind these are the medial and lateral epicondyles (yes, I know, really) and in dogs there's a hole running through the middle of these known as the supratrochlear foramen. Lastly, this part also has a groove like the head, but it is known as the lateral supracondyloid ridge.


If you fancy testing yourself, give this app I made a go!


Radius and Ulna

Next up we have the elbow joint, which is attached to the radius and ulna. Interestingly, these bones cross over each other, so the ulna is proximally attached medially but distally attached laterally, and the radius is the opposite (proximally attached laterally and distally attached medially). In cats, the radius is much rounder at the elbow joint, and this allows for them to pronate and supinate (that is, turn their the right way up or upside down in order to climb).


In the images below, the radius is on the 'left' (cranial) and the ulna is on the 'right' (caudal) except these directions are fairly useless given the crossing over (it just doesn't look like it in the diagram because I'm shite at drawing!)


Starting at the top, the radius has a head and a neck, and sits against the ulna in the semi lunar notch. Speaking of the ulna, it has several strange 'spikes coming off it', one of which looks like the beak of a chicken. That's the anconeal process, and below it is the lateral coronoid process - between these two is the notch. The olecranon sticks out on the opposite side, making the chicken 'head', if you view the anconeal process as a 'beak'! The medial coronoidprocess points towards you in the diagram below, hence why I've only drawn it in the second and not the first, as otherwise it's more confusing than it needs to be!


The first gap between the bones we come across is caused by the radial interosseus crest (in the game below you may get told it's wrong because there's two interosseus crests and I forgot to specify the individual ulna and radius ones!)


Towards the bottom we have the interosseus crest of the ulna causing a gap, and then right at the very bottom we have the medial and lateral styloid processes. There's also the articular circumference.




The Foot

So not the technical term but I don't actually know what that is, so we're going with it! The first thing we see here are the carpal bones, of which we have two rows: the proximal and distal rows.


In the proximal row, we have the radial carpal medially and the ulnar carpal laterally. In the distal row there's 4 tiny bones, not seen in my diagram because they're a bit of a pain to draw (and we haven't technically learnt this yet so trying not to get ahead of myself!) You can see that the ulnar carpal to a certain extent kind of bridges the distal row, and the accessory carpal that we saw above is technically a part of this section. Apparently, it's a good marker for the lateral side, and you can feel it in yourself too!


Then we have the five metacarpals (I've only included 4 here because the first one is a stubby little one and difficult to see from the lateral side!) And finally, there's the phalanges, which have three bones each: proximal, middle, and distal phalanges, ending in the claw! Pretty interesting right?

There are some fairly significant differences between the carnivores and the ungulates (horses, pigs, sheep e.t.c) but this post is already long enough without making me go into that as well! Broadly, things are the same, although size varies massively...maybe that's a post for another day?


Hope you're having a good day and thanks for reading! x

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