ANATOMY OF BIRDS 9 
fowls. These represent red corpuscles which are smaller and larger 
respectively than the average red corpuscle. 
Myelogenous leucocytes appear frequently in young fowls. 
They are like mast cells except that their protoplasm is vacuolated. 
NERVOUS SYSTEM 
The nervous system of birds, with few exceptions, corresponds 
with that of mammals. However, there are several modifications 
which are of considerable interest. The cerebral hemispheres of 
the brain are well developed but have a smooth surface in contrast 
to the convoluted surface observed in the cerebrum of mammals. 
The cerebellum is conspicuous by the smallness of its lateral lobes. 
‘The optic lobes are placed in a lateral inferior position at the 
sides of the cerebellum. The large size of the optic chiasma and 
nerves indicates the paramount importance of the visual apparatus. 
The diameter of an optic nerve is equal to that of the spinal cord in 
its dorsal region. The olfactory lobes are relatively small. The 
hippocampi are absent in the cerebrum of birds. An important fea- 
ture of the nervous system of birds is the absence of nervous tracts 
leading to the spinal cord from the cerebrum. The cerebrum does 
however exert an influence over the rest of the nervous system 
threugh its nervous tracts connecting it with the other divisions of 
the brain. The anatomical evidence of the absence of nervous tracts 
between the cerebral hemispheres and the spinal cord is supported 
by the absence of any marked defects of movement succeeding the 
removal of the cerebrum. The fowl so treated, on recovery from the 
operation, is not sensitive to light or sound. It fails to pick up 
erains and must be fed. Movement is not interfered with as the 
bird constantly walks about aimlessly. Its avoidance of obstacles 
shows that it is guided by some obscure means. Ability to perch 
and to maintain its balance is not lost. The weight of the brain of 
a medium sized fowl is ten grams. 
The spinal cord of birds extends into the coccygeal vertebra. It 
is cylindrical throughout its length except in the sacral and cervico- 
dorsal regions where two expansions are noted. The dorsal columns 
of the cord diverge from each other in the sacrum to form the cavity 
known as the sinus rhomboidalis which represents a ventricular 
dilatation of the spinal canal. The brachial plexus is formed by the 
last cervical and first two dorsal spinal nerves in ducks and geese and 
