In Scheme, is there a built-in function called "ref"?

Question

In Scheme, is there a built-in function called "ref"? I am reading "The Little Learner" and came across the following code:

(define line
  (lambda (x)
    (lambda (θ)
      (+ (* (ref θ 0) x)
         (ref θ 1)))))

(display ((line 7.3) (list 1.0 0.0)))

It gives an error message "variable ref is not bound".

Could you please let me know what is wrong with the above code and how to represent the index of elements in an array?