'To take the reins' definitions:

Definition of 'To take the reins'

From: GCIDE

Rein \Rein\ (r?n), n. [F. r[^e]ne, fr. (assumed) LL. retina, fr. L. retinere to hold back. See Retain.]
1. The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse. [1913 Webster]
This knight laid hold upon his reyne. --Chaucer. [1913 Webster]
2. Hence, an instrument or means of curbing, restraining, or governing; government; restraint. "Let their eyes rove without rein." --Milton. [1913 Webster]
To give rein, To give the rein to, to give license to; to leave withouut restrain.
To take the reins, to take the guidance or government; to assume control. [1913 Webster]