Every kid who compares muscles, knows the biceps. They have 2 heads. This is why they are called "Bi"ceps. One outer belly that is long, another inside that is short. For the artist, it is more important to know where it originates and inserts. Its origin is tucked under the deltoid, so you would not see the "end" of bicep. Its insertion is at the inside of the elbow.
On the opposite side of the arm are the triceps. These extend the arm straight. As the name suggests, there are three muscle bodies. One is pretty much hidden from view while the two form a bit of a V. See the wikipedia image below:
Again, the upper part of the tricep is hidden under the deltoid. The lower part attaches to the elbow.
The last muscle is the "boring" muscle. After you erase the bulging biceps and the triceps behind the arm. You still have something mass of the arm. This is like a slim lower mass under the bicep and close to the elbow. It is the brachialis. You can see it below near the biceps. In fact, this muscle is responsible for a lot of power. For the artist, though, keeping it mind helps add a little bit of definition and realism to what might look like a balloon animal arm.
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