Hey, you're watching Proko. My name is Stan Prokopenko.
As you might of guess from that disturbing intro
This episode is about the Pectoralis Major muscle
Even though male and female chest look different, both have the same muscles.
On a female the pectoralis major is still visible under the breast tissue
What and Where?
The pectoralis major is like a very flat box, tucked to the corner of the clavicle and sternum
That converges toward the humerus
It shape is simple but gets a little tricky
because the muscle fibers overlap in a folding fan pattern
Lets start with the basics
The pecs originates from medial half of the clavicle's front plane,
the entire length of the sternum, and top of the external oblique's aponeurosis
The pectoralis inserts into the humerus at the bicipital groove
which is a deep, oblique groove across the upper third of the humerus
The pectoralis major doesn't insert into one small point
it attaches with a tendon that's as wide as your wrists
The anterior deltoid head passes over the insertion, and covers the tendon
The pectoralis major has not one, but four functions
all pertaining to shoulder movement
The pec pulls the arm forward, like for a high five
Interestingly, the pec also helps to lower the arm
It can also adduct the arm sideways toward the body, like a slap
and rotate the arm medially
like for arm wrestling
That's right, arm wrestling involves your chest
but, how can the pecs do so many different things?
The answer is in the anatomical structure
The pectoralis major is structured like a folding fan
The muscle bundles radiate out from the armpit
They actually cross over each other where the pecs lift off the rib cage
This twisting means that the lowest bundles of the chest
insert the highest on the humerus and vice versa
There are three key sections
The clavicular portion originates, unsurprisingly, on the clavicle.
It's a narrow triangle that angles downward towards the insertion
It's also the thickest portion and it can obscure the front plane of the clavicle
This is why you would usually just see the top plane of the clavicle on a muscular body type.
On a leaner person, the muscle us thin enough that you do see the front plane of the clavicle
The pec is separated from its clavicular neighbours, the deltoid, by the infraclavicular fossa
That's a fancy word for the little triangular groove between the two muscles
On the surface, the clavicular portion appears to stretch the farthest to the arm.
You'll see it rolling over the other sections just before it disappears under the deltoid
The other two portion of the pec go under this clavicular portion
The sternum portion is the largest of the three.
It's also very thin, and you can sometimes see the ribs through it
You can distinguish the ribs from pecs by their consistent, horizontal angle
Pectoralis bundles are more obliques and their angles radiate
Be aware on how the two pecs share the sternum
In the top half, the sternal bundles originate very close to each other
sometimes with no visible distance
On the lower half, the pecs insert closer to the edge of the sternum
so there's a triangular depression there
But this also varies
The gap between the pecs can be wild and crazy, like a rubin vase
or it can be a straight line down with a narrow gap
You pick which fits your character and picture better
Lastly is the abdominal portion
This is the deepest and smallest layer of the pecs
pointing up laterally to the insertion
It originates from the aponeurosis of the external oblique
along the level of the fifth rib
Remember from the last lesson that an aponeurosis is a large, thin tendon
The aponeurosis of the external oblique covers the rectus abdominis
And the abdominal portion of the pec attaches to the very top of it
On some people, this section is clearly distinct and clearly lower than the other two sections
And on other people, it's totally buried
genetics
And much like the cherry on top of the proverbial ice cream
The nipple is the last thing to go on your man chest sunday
The nipple is not a consistent, since fat can sag the skin and move the whole nipple lower
But, they're about one head measurement from the chin
If you draw a 45 degree angle out from the pit of the neck
It will line up with the nipple and the edge of the serratus muscle
In the next video, we'll take a look at how you can use all these information
to construct the form of the pecs for your drawings
Variations
There are variations of the chest, depending on the physique of the person
such as lean, average, muscular, and heavy
to learn more about these variations, head on to proko.com/anatomy
The premium version has longer lessons
3d models
and, additional demonstrations
... just sayin'
proko.com/anatomy
Assignment
Your homework is a two parter
from part one, do a tracing over a model photo of the pectoralis major
and all of its bundles
If your not sure how to do a tracing refer back to the "How to Do an Anatomy Tracing" lesson
For part two of the assignment,
invent the pectoralis major on top of photos of Skelly
I've provided reference photos for part one and two in the link below
Download those and start drawing
If you have the skelly app, you can draw your own poses and draw the pecs on top
Post your work in our anatomy group at proko.com/groups
Thanks for watching everybody!
Do the chest bump worthy thing and share this video with your friends
Give me a thumbs up and comment below
And if your looking for the Proko news, look no further from the Proko news letter!
At proko.com/subscribe
See you next time