THE 4IR-SACS 391 



Each communicates with the outer and posterior 

 angle of the corresponding lung. 



3. The anterior thoracic air-sacs cover the ventral surfaces 



of the anterior two-thirds of the lungs, and extend 

 back so as to overlap the posterior thoracic sacs. 

 They lie at the sides of the thorax, and are in exten- 

 sive contact with the ribs and with the pericardium. 

 Each communicates with the ventral surface of 

 the lung, rather in front of the middle of its length, 

 and not far from the median plane of the body. 



4. The interclavicular air-sac is a median sac, formed by 



the fusion of two originally distinct ones. It consists 

 of, (1) a median portion, which surrounds the hinder 

 end of the trachea, and can readily be seen in the 

 angle between the two clavicles; and (2) lateral 

 processes, one of which runs forwards along each 

 side of the trachea, while others pass outwards, 

 beneath the coracoid, and open into large axillary 

 air- sacs in the neighbourhood of the shoulder- 

 joints, which communicate with the cavities of the 

 humeri. 



The interclavicular air- sac opens into the ventral 

 surface of each lung, immediately in front of the point 

 of entrance of the main bronchus. 



5. The cervical air- sacs are much smaller, and lie in front 



of the lungs at the base of the neck, alongside the 

 vertebral column. 



They communicate with the anterior angles or 

 apices of the lungs. 



IV. DISSECTION OF THE DIGESTIVE SYSTEM. 



A. The Buccal Cavity, 



Open the mouth slightly : cut through the angle of the 

 mouth, parallel to the jaws, for about half an inch on each 

 side ; and turn down the floor of the mouth so as to expose the 

 cavity fully. Note the following structures. 



