364 CHORD AT A. 



they are present in certain fossils. The large crop is use< 

 for the storage of quantities of grain. The pigeon ha 

 many enemies and has to fill its crop when the occasio] 

 presents itself. In the crop the food is partially softened 

 and is passed gradually into the stomach which secrete 

 a digestive fluid. It is then passed into the gizzard ii 

 which it is ground and crushed to pieces. There are alway 

 present in the gizzard a number of small fragments of stone 

 which, churned together with the food by the muscular wall 

 of the stomach, reduce the grain to small pieces. Thenc 

 they pass into the duodenum and ileum in which absorptio] 

 is effected. It will be seen that there is no gall-bladder ii 

 the pigeon, but this is present in closely allied birds. 



The coelom is mainly represented by the large abdomina 



C lom cavity and the smaller pericardial cavity aroun( 

 the heart. The two cavities are, as in the frog 

 completely separated from each other. 



The alimentary canal is suspended by a dorsal mesenter; 

 in which run the blood-vessels, as in most vertebrates. I 

 median ventral mesentery attaching the liver to the sternun 

 is termed tho, falciform ligament. 



The heart is proportionately very large ; it lies imme 

 diately in front of the liver, and is four-chambered. Th^ 



Blood- single ventricle of the lower types is here divide( 

 into two by a septum. Hence there is a lef 

 ventricle communicating wdth the left auricL 

 and a right ventricle communicating with the right auricle 

 The supply of blood to the auricles is similar to that c 

 the frog, i.e.^ venous blood from the system comes bad 

 to the right auricle and arterial blood from the lung 

 comes back to the left auricle. On contraction eacl 

 auricle empties its blood into the ventricle of the sami 

 side through the auriculo-ventricular valves. On con 

 traction of the ventricles the left sends its blood to th( 

 system and the right to the lungs. Hence the two current 

 are quite apart throughout their course, and the right sid< 

 of the heart acts as a respiratory heart, the left side per 

 forming the part of a systemic heart. 



If a section be made across the posterior half of th( 

 heart, the two ventricles will be seen. The left ventricula 

 cavity is small and has very thick walls ; the right ii 



