RESPIKATOEY ORGANS. 305 



hence their size. We, however, with a smaller soft palate, can 

 breathe through our mouth. The nasal fossae are lined by a 

 sensitive mucous membrane, part of which is olfactory in 

 function. Closely connected with the nasal cavities are a 

 number of winding spaces excavated in the substance of the 

 bones of the head, on the limits of the cranium and face, and 

 around the ethmoidal masses which they envelop. These 

 cavities form the so-called sinus (Fs), which increases the area of 

 the skull for the attachment of muscles and for lightening its 

 weight. In the ox the frontal sinus is prolonged into those 

 bony cores that support the horns. Opening from the back 

 of the throat is a slit-like aperture, the glottis, partly protected 

 by a flap-like portion, the epiglottis (Ep). On each side of the 

 opening are placed internally the two vocal cords. These two 

 elastic bands project within the larynx (L), and between them 

 include the space forming the glottis. 



The larynx is the organ of voice, and it also admits air 

 during respiration. It is practically a cartilaginous box 

 flattened on each side. Entering into its composition are 

 five pieces of cartilage, numerous muscles, nerves, blood-vessels, 

 and a lining of mucous membrane. This organ dilates and 

 contracts. When paralysis sets in, causing very rapid move- 

 ments, it produces the so-called "roaring'' in horses. 



The trachea (Tr) is a flexible tube arising from the base of 

 the larynx, and is supported by a series of incomplete C-shaped 

 cartilaginous rings. It passes between the two first ribs and 

 enters the chest, arriving above the left auricle to the right 

 of the big blood-vessel, the posterior aorta, where it bifurcates, 

 forming the two bronchi. Each bronchial tube divides up 

 in the lungs, sending out an innumerable number of small 

 branches, resembling a tree embedded in the substance of the 

 lung. 



The thorax or chest,, in which the lungs and heart are 

 situated, forms a cavity shut in by the ribs, backbone, diar 

 phragm, and sternum. The thorax is lined by two serous mem- 



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