136 



GENEBAL ORNITHOLOGY. 



whence arise all the nerves of the body, excepting those of the sympathetic nervous system). 

 The lower ring is the htemal arch (Gr. alfia, 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 



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lungs. Such 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-barrelled 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 vertebra;, in the trunk ; and in the head certain 



