a. [a. F. crucial (Paré 16th c.), f. L. cruc-em cross + -AL.]
1. (Chiefly Anat.) Of the form of a cross, cross-shaped, as crucial incision; spec. the name of two ligaments in the knee-joint, which cross each other in the form of the letter Ｘ, and connect the femur and tibia; also applied to ‘the transverse ligament of the atlas and its upper and lower offshoots combined’ (Syd. Soc. Lex.).
1706. Phillips (ed. Kersey), s.v. Incision, Crucial Incision, the cutting or lancing of an Impostume or Swelling cross-wise.
1767. Gooch, Treat. Wounds, I. 451. Making an incision quite cross to the bone, from ear to ear; which section is preferable to the crucial, commonly made.
1804. Abernethy, Surg. Obs., 256. Between the condyles of the os femoris and the crucial ligaments.
1859. J. Tomes, Dental. Surg., 338. In the molar teeth of the lower jaw, the decay sometimes takes a crucial shape.
1861. S. Thomson, Wild Fl., III. (ed. 4), 302. The crucial flowers.
2. That finally decides between two rival hypotheses, proving the one and disproving the other; more loosely, relating to, or adapted to lead to such decision; decisive, critical.
This sense is taken from Bacon’s phrase instantia crucis, explained by him as a metaphor from a crux or finger-post at a bivium or bifurcation of a road. Boyle and Newton used the phrase experimentum crucis. These give ‘crucial instance,’ ‘crucial experiment,’ whence the usage has been extended. Occasionally the sense intended seems to be ‘of the nature of a crux or special difficulty’; see CRUX.
[1620. Bacon, Nov. Org., II. xxxvi. Instantias Crucis: translato Vocabulo a Crucibus, quæ erectæ in Biuijs, indicant & signant viarum separationes. Has etiam Instantias Decisorias & Iudiciales, & in Casibus nonnullis Instantias Oraculi, & Mandati appellare consueuimus.
1672. Newton, Light & Colours, i. The gradual removal of these suspicions at length led me to the Experimentum Crucis.] [Not in Johnson, Todd, or Webster 1828.]
1830. Herschel, Stud. Nat. Phil., II. vi. 150. What Bacon terms ‘crucial instances,’ which are phenomena brought forward to decide between two causes, each having the same analogies in its favour.
1869. J. Martineau, Ess., II. 134. Crucial experiments for the verification or correction of his theory?
1874. Helps, Soc. Press., xvi. 226. Showing where, at some crucial point of the story, fraud or delusion might enter.
¶ 3. Apparently associated with the trying action of a ‘crucible.’
1856. Mrs. Browning, Aur. Leigh, V. 310. And from the imagination’s crucial heat Catch up their men and women all a-flame For action.
1860. Lit. Churchman, VI. 222/1. This crucial time … which will purge out the dross and tin of popery and dissent.
Hence Crucially adv., in a crucial manner.
1879. H. Grubb, in Trans. R. Dubl. Soc., 188. Any one can try this crucially for himself.
