The pyridine derivatives of general formula I, wherein B is other than oxygen and sulphur atoms, Z represents an ethynylene group, R1 represents a grouping of the formula: --COOR4, in which R4 represents a hydrogen atom, or an alkyl group containing from 1 to 12 carbon atoms, and the other symbols are as hereinbefore defined, i.e. compounds of the general formula: ##STR16## (wherein the various sy