Empirically considered, there is a striking difference between the correct vocal action and any other manner of tone-production. A perfect vocal tone awakens in the hearer a distinct set of auditory and muscular sensations. Attentively observed, the muscular sensations of the hearer indicate that the perfect vocal tone is produced by the balanced and harmonious action of all the muscles of the singer's vocal mechanism. In listening to perfect singing the hearer feels that every muscle of the singer's vocal organs is contracted with exactly the appropriate degree of strength. Any vocal tone of unsatisfactory sound awakens in the hearer a set of muscular sensations, the direct opposite of those indicating the correct vocal action. An incorrectly produced tone imparts to the hearer a sensation of stiffness and undue muscular tension, located more or less definitely in the throat. This sensation indicates that the singer's throat is stiffened by excessive muscular contraction. Further, this feeling of throat stiffness indicates to the hearer that the singer's vocal action would become correct if the undue muscular tension were relaxed. Combining now the results of empirical and scientific investigation of the voice, throat stiffness is seen to be the interfering influence which disturbs the instinctive connection between voice and ear. Let us now consider the meaning of throat stiffness as a feature of incorrect tone-production. First, what is muscular stiffness? All the voluntary muscles of the body are arranged in opposed pairs, sets, or groups. A typical pair of opposed muscles are the biceps and triceps of the upper arm. Contraction of the biceps flexes the forearm at the elbow; the contrary movement, extending the forearm, results from the contraction of the triceps. This principle of opposition applies to the entire muscular system. One set of muscles raises the ribs in inspiration, another set lowers them in expiration; one group flexes the fingers and clenches the fist, an opposed set extends the fingers and opens the hand. Muscular opposition does not imply that the entire structure is made up of parallel pairs of muscles, like the biceps and triceps, located on opposite sides of the same bone. It means only that the opposed sets pull in contrary directions. Each opposed set consists of muscles of about equal strength. Under normal conditions of relaxation the entire muscular system exerts a slight degree of contraction. To this normal state of oppositional contraction the name "muscular tonicity" is given. The present purpose does not call for a discussion of the subject of muscular tonicity. This form of contraction has no direct bearing on the performance of voluntary movements.