THE RABBIT 297 



The Respiratory System. This system consists of the nasal 

 cavities, the nasopharynx, the pharynx, the trachea with the 

 larynx, the bronchi, and the lung 



The nasal cavities are two in number and lie side by side in the 

 anterior portion of the head, separated by a median partition, the 

 nasal septum. They communicate with the outside by the nostrils, 

 or external nares. and with the nasopharynx by the internal nares. 

 Cut away the soft palate and expose the nasopharynx and the 

 internal nares. Note a pair of slits in the lateral walls near the 

 middle of the nasopharynx : these are the openings of the Eusta- 

 chian tubes, which join the middle ear with the pharynx. 



The nasal cavities are rendered very irregular in outline by 

 projections of their walls called the turbinals. which serve to in- 

 crease the sensory surface : they will be studied with the skeletal 

 system. 



Make a transverse section of the anterior end of the nose, using 

 a large scalpel. The cartilaginous nasal septum will be seen be- 

 tween the two nostrils, and also the anterior turbinals. 



Exercise 23. Draw the cross section. 



Lay bare and study the trachea and the larynx, and the organs 

 lying near them. Just dorsal to the trachea is the oesophagus, the 

 muscular tube which extends from the pharynx to the stomach. 

 On each side of the trachea are the carotid artery and the internal 

 jugular vein, and also a nerve trunk which is composed of the 

 vagus and sympathetic nerves closely bound together : these 

 nerves must not be cut. Near the anterior end of the trachea is 

 the thyroid gland. 



Dissect from the lower jaw the tongue, remove it with the 

 larynx, the trachea, and the lungs from the body, and place them 

 in a pan of water. The hyoid bones may be felt at the base of 

 the tongue : carefully dissect away the muscular tissues of the 

 tongue and expose these bones. They consist of a transverse 

 median bone, the basihyal. and two pairs of short processes called 

 the horns or cornua : note their relation to the larynx. 



Study the larynx, first removing the muscles from its sides and 

 cutting it open by a mid-dorsal slit. The glottis, the opening into 



