The medicine is absorbed through that, although not as fast as when it's put directly into the vein. When it's injected, it's squeezed into the muscle tissue until it gets absorbed. When something is put into the bloodstream directly instead of into the muscle, it acts almost immediately because it is quickly distributed evenly through the body. When it goes into the muscle, it is more slowly absorbed into the bloodstream, where it is then distributed through the body. When you put it directly into the bloodflow, it just cuts out the middle man and acts much faster.
Some drugs just have to kick in and act faster than others.
Let me know if there's anything else you want to know. Son is fine. Consequently, he practically tore his nail off his big toe, which, of course, he ignored until it got infected. But his other foot hurts now because he was walking on it funny to favor the first foot. He'll do fine with a little ice, motrin and rest for the bad ankle and the toe will survive.