"The power of life exists in every part of a person's body—even in his toenails.
However, there can be no doubt that this power manifests to a different extent in different organs—for instance, in the brain as opposed to the hand or foot.
And that difference is not only quantitative but qualitative as well—to an incomparable degree.
Rabbi Schneur Zalman of Liadi distinguishes between the external and internal organs.
The external organs are the 248 parts of the body that have bones.
The internal organs are the brain, heart, liver, and so forth.
From a halakhic point of view, these latter are not counted among the 248 organs or limbs.
Yet they possess a more inward and inclusive life force than the 248 limbs.
And so Torah learning, which relates to the brain, is in a way superior to all the other mitzvot and equals all the other commandments combined.
The Baal HaTanya teaches that a person fulfills the mitzvah of Torah study by reciting the holy words aloud.
This involves a physical action—in that regard, Torah is like other mitzvot.
But in addition, there is a mitzvah of knowledge, of understanding what one is learning.
And the two are not necessarily alike.
A person can fulfill the mitzvah of Torah study without knowing Torah.
Contrarily, he can know Torah without at that moment engaging in the mitzvah of Torah study.
Like any other mitzvah, Torah study requires some connection to the physical world.
Thus, it can exist only within the garment of the 248 limbs.
On the other hand, the knowledge of Torah, which does not possess a physical garb, is comparable to one of the inner organs, which are not considered one of the 248 limbs of the body yet on which the life force of the entire body depends.
Torah knowledge thus corresponds to the life of the soul that energizes the body."
--Rabbi Adin Steinsaltz
From Understanding the Tanya by Rabbi Adin Steinsaltz