136 
GENERAL ORNITHOLOGY. 
whence arise all the nerves of the body, excepting those of the sympathetic nerv^ous system). 
The lower ring is the hcemal arch (Gr. al^a, haima, blood), which similarly contains a section 
of the principal blood-vessels and viscera. Fig. 55 shows such a section, made across the 
thoracic or chest-region of the trunk. Here the upper ring (neural) is contracted, only sur- 
rounding the slender spinal cord, while the lower ring is expanded to enclose the heart and 
lungs. Sach a section, made in the region of the skull, would show the reverse ; the upper 
ring greatly inflated to contain the brain, the lower contracted and otherwise greatly modified 
into bones of the jaws. Thus the trunk of a vertebrate is a double-ban-elled tube ; one tube 
above for the nervous system, the other below for the viscera at large ; the partition between 
the two being a jointed chain of solid bones from one end of the body to the other. These 
solid bones are the centrums or bodies of vertebrce, in the trunk ; and in the head certain 
