This lump was called _Hippomanes_; which also more truly designated,
according to Virgil, another thing. The following paragraphs from Mr.
Keightley's excellent _Notes on Virgil's Bucolics and Georgics_ will fully
explain both meanings:
"_Hippomanes_, horse-rage: the pale yellow fluid which passes from a
mare at that season [_i. e._ when she is horsing] (cf. _Tibul._ II. 4.
58.), of which the smell (_aura_, v. 251.) incites the horse.
"_Vero nomine._ Because the bit of flesh which was said to be on the
forehead of the new-born foal, and which the mare was supposed to
swallow, was called by the same name (see _Æn._ IV. 515.); and also a
plant in Arcadia (_Theocr._ II. 48.). With respect to the former
Hippomanes, Pliny, who detailed truth and falsehood with equal faith,
says (VIII. 42.) that it grows on the foal's forehead; is of the size
of a dried fig (_carica_), and of a black colour; and that if the mare
does not swallow it immediately, she will not let the foal suck her.
Aristotle (_H. A._, VIII. 24.) says this is merely an old wives' tale.
He mentions, however, the [Greek: pôlion], or bit of livid flesh, which
we call the foal's bit, and which he says the mare ejects before the
foal."--_Notes, &c._, p. 273. on _Georgic._ III. 280. ff.
With regard to the plant called _Hippomanes_, commentators, as may be seen
from Kiessling's note on Theocritus, ii. 48., are by no means agreed.
Certainly Andrews, in his edition of Freund, is wrong in referring Virgil
_Georgic._ III. 283. to that meaning. The use of _legere_ probably misled.