_The larynx_ may be compared to a box open at both ends. The several cartilages that form it are united by ligaments. It is lined by a mucous membrane. The posterior extremity is united to the first cartilaginous ring of the trachea. The anterior opening is closed by the epiglottis. Just within is a V-shaped opening that is limited laterally by the folds of the laryngeal mucous membrane, the vocal chords. _The trachea_ is a cylindrical tube originating at the posterior extremity of the larynx, and terminating within the chest cavity at a point just above the heart in the right and left bronchial tubes. It is formed by a series of cartilaginous rings joined together at their borders by ligaments and lined by a mucous membrane. _The bronchial tubes_ resemble the trachea in structure. They enter the lungs a short distance from their origin, where they subdivide into branches and sub-branches, gradually decreasing in calibre and losing the cartilaginous rings, ligaments and muscular layer until only the thin mucous membrane is left. They become capillary in diameter, and finally open into the infundibula of the air cells of the lungs. _The lungs_ take up all of the space in the thoracic cavity not occupied by the heart, blood-vessels and oesophagus. This cavity resembles a cone in shape that is cut obliquely downwards and forward at its base. The base is formed by the diaphragm which is pushed forward at its middle. It is lined by the pleura, a serous membrane, that is inflected from the wall over the different organs within the cavity. The median folds of the pleura divide the cavity into right and left portions. A second method of describing the arrangement of the pleura is to state that it forms two sacks, right and left, that enclose the lungs. The lungs are the essential organs of respiration. The tissue that forms them is light, will float in water, is elastic and somewhat rose-colored. Each lung is divided into lobes, and each lobe into a great number of lobules by the supporting connective tissue. The lobule is the smallest division of the lung and is formed by capillary bronchial tubes, air cells and blood-vessels. It is here that the external respiration or the exchange of gases between the capillaries and the air cells occurs. VENTILATION.--It is agreed by all persons who have investigated the subject, that unventilated stable air is injurious to animals. At one time it was believed that the injurious effects resulting from the breathing of air charged with gases and moisture from the expired air and the animal's surroundings, were due to a deficiency in oxygen. It is now believed that the ill-effects are mainly due to the stagnation of air, the humid atmosphere, and the irritating gases emanating from the body excretions.