But the French t, d, s, z are true Dentals, and the Latin t, s, d, z may have been the same.
Labo-Dentals
§ 43. When the lower lip is pressed against the upper teeth, and the current of air forces its way out between them, we get the Labio-Dental Fricatives, Breathed as f m Eng. firm, Lat. firmus; Voiced as Eng. v in van.
But note that this latter sound did not arise in Latin from Consonant u until the 2nd Century A.D. (see § 158). In Latin at the time of Cicero Consonant u (which in modern times has in Latin texts been commonly printed v) was pronounced very nearly as English w, that is to say, it was not Labio-Dental but a pure Labial (§ 45).
Labials
§ 44. Labial Plosives are made when the current is stopped by closing the lips, p (Breathed), b (Voiced).
§ 45. Pure Labial Fricatives are made by rubbing the current between the lips, Eng. wh in what (Breathed), w in wet (Voiced); note however that in vulgar southern English wh is sounded as w. The only difference between this Voiced Labial Fricative (Eng. w) and consonant u (Lat. v as in vento) is that in the Fricative the lips are held a little longer and more tightly so that the rubbing is more severe.
