364 CHORDATA. 
they are present in certain fossils. The large crop is used 
for the storage of quantities of grain. The pigeon has 
many enemies and has to fill its crop when the occasion 
presents itself. In the crop the food is partially softened, 
and is passed gradually into the stomach which secretes 
a digestive fluid. It is then passed into the gizzard in 
which it is ground and crushed to pieces. There are always 
present in the gizzard a number of small fragments of stones 
which, churned together with the food by the muscular walls 
of the stomach, reduce the grain to small pieces. Thence 
they pass into the duodenum and ileum in which absorption 
is effected. It will be seen that there is no gall-bladder in 
the pigeon, but this is present in closely allied birds. 
The ccelom is mainly represented by the large abdominal 
cavity and the smaller pericardial cavity around 
the heart. The two cavities are, as in the frog, 
completely separated from each other. 
The alimentary canal is suspended by a dorsal mesentery 
in which run the blood-vessels, as in most vertebrates. A 
median ventral mesentery attaching the liver to the sternum 
is termed the fakiform ligament. 
The heart is proportionately very large; it les imme- 
diately in front of the liver, and is four-chambered. The 
single ventricle of the lower types is here divided 
into two by a septum. Hence there is a left 
ventricle communicating with the left auricle 
and a right ventricle communicating with the right auricle. 
The supply of blood to the auricles is similar to that of 
the frog, z.e., venous blood from the system comes back 
to the right auricle and arterial blood from the lungs 
comes back to the left auricle. On contraction each 
auricle empties its blood into the ventricle of the same 
side through the auriculo-ventricular valves. On con- 
traction of the ventricles the left sends its blood to the 
system and the right to the lungs. Hence the two currents 
are quite apart throughout their course, and the right side 
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 the 
heart, the two ventricles will be seen. The left ventricular 
cavity is small and has very thick walls; the right is 
Colom. 
Blood- 
Vascular. 
