136 



GENERAL ORNITHOLOGY. 



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

 The lower ring is the hcemal arch (Gr. ai/ia, liaima, blood), which similarly contains a section 

 of the principal blood-vessels and viscera. Fig. 55 shows siidi 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 tlie licart 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 vertebrae, in the trunk ; and in the head certain 



