_Larynx._—The upper end of the trachea is modified into the organ of voice or “larynx,” the air passing through which to and from the lungs is made use of to set the edges of the “vocal cords,” or fibrous bands stretched one on each side of the tube, into vibration. The larynx is composed of several cartilages, such as the “thyroid,” the “cricoid,” and the “arytenoid” which are moved upon one another by muscles, and suspended from the hyoidean arch. By alteration of the relative position of these cartilages the cords can be tightened or relaxed, approximated or divaricated, as required to modulate the tone and volume of the voice. A median tongue-shaped fibro-cartilage at the top of the larynx, the “epiglottis,” protects the “glottis,” or aperture by which the larynx communicates with the pharynx, from the entry of particles of food during deglutition. The form of the larynx and development of the vocal cords present many variations in different members of the class, the greatest modification from the ordinary type being met with in the Cetacea, where the arytenoid cartilages and epiglottis are united in a tubular manner, so as to project into the nasal passage, and, being grasped by the muscular posterior margin of the palate, provide a direct channel of communication from the lungs to the external surface. An approach to this condition is met with in the Hippopotamus and some other Ungulates; it is indeed so general as an abnormality, that Howes suggests that an internarial epiglottis may have been a primitive feature common throughout the class. Nearly all mammals have a voice, although sometimes it is only exercised at seasons of sexual excitement. Some Marsupials and Edentates appear to be quite mute. In no mammal is there an inferior larynx, or “syrinx,” as in birds. _Diaphragm._—The thoracic cavity of mammals differs from that of the Sauropsida in being completely separated from the abdomen by a muscular partition, the “diaphragm,” attached to the vertebral column, the ribs, and the sternum. This is much arched, with the convexity towards the thorax, so that when its fibres contract and it is flattened the cavity of the thorax is increased, and when they are relaxed the cavity is diminished.