To what do these eighteen benedictions correspond? R. Hillel the son of Samuel b. Nahmani said: To the eighteen times that David mentioned the Divine Name in the Psalm, Ascribe unto the Lord, O ye sons of might.15 R. Joseph said: To the eighteen times the Divine Name is mentioned in the Shema'. R. Tanhum said in the name of R. Joshua b. Levi: To the eighteen vertebrae in the spinal column.and then:
These eighteen are really nineteen? — R. Levi said: The benediction relating to the Minim18 was instituted in Jabneh.19 To what was it meant to correspond? — R. Levi said: On the view of R. Hillel the son of R. Samuel b. Nahmani,20 to The God of Glory thundereth;21 on the view of R. Joseph, to the word 'One'22 in the Shema'; on the view of R. Tanhum quoting R. Joshua b. Levi, to the little vertebrae in the spinal column.Thus, there are eighteen or nineteen vertebrae in the spinal column. This does not seem, at first glance, to correspond to modern science, nor to what we know of contemporary science. For reading on this subject, see here, here and here. It might just be a matter of which bones they counted and which they did not.
One approach, which seems iffy to me: