ALIMENTARY SYSTEM 



407 



or proventriculus, which has glands which secrete the gastric 

 juice, and then the gizzard or muscular stomach, a lens-shaped 



Fig. 321. — A pigeon opened from the ventral side to show the principal organs 



in their natural positions. 



a.th.s.. Anterior thoracic air sac ; ab.s., abdominal air sac ; ao.a., aortic arch ; br.a., brachial artery ; c.c, 

 common carotid artery ; cl.o., cloacal opening ; cr., crop ; dm., duodenum ; inn., innominate arteries ; 

 /., lung ; Iv., left ventricle ; Ir., liver ; ms., muscles concerned in the movements of the syrin.x ; oes., 

 CESophagus ; p.m., pectoralis major muscle ; p.th.s., posterior thoracic air sac ; pcm., pericardium (torn 

 open) ; pes., pancreas ; pct.a., pectoral artery ; r.v., right ventricle ; scl.a., subclavian artery ; thm., 

 thymus ; tra., trachea. The gizzard is seen opposite the forceps which hold back the abdominal wall. 



chamber with very thick muscular walls and a horny lining, 

 where the food is ground up by the aid of small stones which have 

 been swallowed. It lies below the proventriculus, which opens 

 on its dorsal border, rather to the left side : on the right side 



