3. The resonating cavities--trachea, pharynx, mouth, and the cavities of the nose and head. 4. The organs of articulation--the tongue, lips, teeth, etc. Each of these organs performs a different function in the production of useful and beautiful sounds. It is the generally accepted view among vocal scientists that each portion of the vocal mechanism has one and only one correct mode of operation. Many forms of incorrect action are also possible; indeed, the untrained voice is believed to be liable in almost every case to faulty operation in some respect. Scientific investigation of the voice has for its purpose the determining of the correct muscular actions of the various parts of the vocal mechanism, and the formulation of exercises whereby the student is enabled to acquire command of these actions. Modern methods of voice culture embody the results of scientific study of the vocal mechanism. It must be observed that not all investigators are agreed upon what constitutes the correct vocal action. Several conflicting theories have been urged concerning the proper movements of the breathing and laryngeal muscles and the operations of the resonating cavities. An exhaustive treatment of these various theories is, however, not called for here. Some of them have been discarded, others have but few followers. The majority of authorities are in fair agreement concerning the theoretical basis of voice culture. In the present chapter only the theoretical aspect of the subject will be considered; the third chapter will be devoted to the practical methods which embody the doctrines of vocal science. Two opposed sets of muscles are concerned in the operations of breathing, those which, respectively, fill and empty the lungs. The action of inspiration consists of an expansion of the chest cavity, which by increasing its cubical capacity draws in air from the outside to fill what would otherwise be a vacuum. The chest cavity is conical in shape, its base being formed by the diaphragm, and its sides and apex by the ribs, the breast-bone, and the intercostal muscles. Broadly speaking, there are two distinct forms of muscular action by which the chest cavity can be expanded in inspiration: one is the sinking of the base of the chest cavity, the other the broadening of the chest by raising the ribs. Either of these can with some little practice be performed independently of the other.