Service of God is not limited to the sacrificial rites in the Temple; it includes the service of the heart, which is prayer. There are three essential stages of prayer: praising God, petitioning Him that He provide for one’s needs, and thanking Him for the good one has received.

It is a positive commandment to pray every day, as it is stated: “You shall serve the Lord your God” (Exodus 23:25). The Sages learned through an oral tradition that this service is prayer. It is also stated: “To serve Him with all your heart” (Deuteronomy 11:13). The Sages said: “What is service of the heart? It is prayer”…. The requirement imposed by this commandment is as follows: A person must pray and supplicate every day. He should recite praises of the Holy One, blessed be He; then request the provision of his needs, using [language of] request and supplication; and then give praise and thanks to God for the good that He has bestowed upon him. Each person [should pray in this fashion] according to his abilities. (Rambam, Mishneh Torah, Laws of Prayer 1:1–2)

Despite its centrality in the prayer service, the text of the Amida prayer, also known as Shemoneh Esreh (literally, “eighteen,” because it was originally formulated with eighteen blessings), is of rabbinic, not biblical, origin. Its importance is compared to that of the spinal cord, which, although it is not counted as one of the limbs of the body, is what allows the entire body to function. Similarly, prayer is the foundation and facilitator of all of the commandments.

The number of blessings in the Shemoneh Esreh prayer corresponds to the number of vertebrae in the spinal column:

For the eighteen blessings of the Shemoneh Esreh correspond to the eighteen vertebrae of the spinal column that house the spinal cord, as is known from the Zohar (Tikkunei Zohar 18) and from the Talmud in Berakhot (28b). By way of analogy, a person has 248 limbs, including the eighteen vertebrae of the spinal column. But the spinal cord itself, which runs through the vertebrae, is not counted among the number of limbs. Nonetheless, it is what supports and sustains all the limbs. The spinal cord runs from the head to the thighs, and the life force is drawn through it from the brain to all of the limbs. For the limbs are connected to the ribs, the ribs to the vertebrae, and the vertebrae to the spinal cord. The spinal cord is not one of the limbs, but it is what connects them, starting from the brain and spreading to the legs. Therefore, if the spinal cord is damaged, one’s life is in danger.

Intent in prayer corresponds to the spinal cord:

This can be likened to the commandments, which are the 248 limbs of the King. Our Sages, of blessed memory, said: “Fulfilling the commandments requires intent” (Berakhot 13a). “Intent” refers to the element of prayer, which serves as the “intent” and inner content of the commandments. Prayer is the primary [factor] that supports and stabilizes the 248 positive commandments. It is comparable to the spinal cord, which supports and sustains the limbs, even though it is not counted among the 248 limbs. (Rabbi Shneur Zalman of Liadi, Likutei Torah, Balak, s.v. Lo hibit aven)

Further reading: For more on the institution of prayer and its source, see A Concise Guide to the Sages, p. 469.