202 GENERAL ORNITHOLOGY part n 



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 surrounding the slender spinal cord, while the 

 lower ring is expanded to enclose the heart and 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 con- 

 tracted 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 axial 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 soM 

 bones are the centrums or bodies of mrtebrm, in the trunk ; and in the 

 head certain bones which in some respects correspond with the 

 centrums of vertebrae. The entire chain or series of vertebrae com- 

 poses the back-bone or spinal column ; with its connections (thorax) 

 and anterior continuation (skull) it is the axial skeleton. The skull 

 is considered by some competent anatomists to consist of modified 

 vertebrae. The skull-bones have certainly the position and relations 

 of parts of vertebrae ; to a certain extent they resemble vertebrae, 

 as in being divisible into several segments, like as many vertebral 

 segments ; they are also directly in the axis of the body, enclosing 

 a part of the cerebro-spinal nervous system above, and portions of 

 the visceral systems below. But supposed strict morphological 

 correspondence of cranial bones with vertebrae is not supported by 

 their mode of development, and is now generally denied, the relation' 

 being considered rather analogical and physiological than homolo- 

 gical and morphological. 



1. THE SPINAL COLUMN 



A Vertebra (so called from the flexibility of the chain of 

 vertebrae; Lat. vetio, I turn) consists of a solid body or centrum, 

 and more or fewer processes or apophyses, some of which have 

 separate ossific centres. Plate-like processes which arch upward 

 from either side of a centrum to enclose the neural canal are the 

 newal a/rches or neurapophyses (Fig. 54, n, n) ; at their union in the 

 middle line above they commonly send up a process called the 

 neural spine (ns). Transverse processes from the sides of the neural 

 arch are diapophyses (Gr. Sta, dia, across ; Figs. 54, 55, d, d). ObUque 

 processes from the sides of the same arches, serving to lock them 

 together, are zygapophyses (Gr. fijyov, mgon, a yoke ; Fig. 55, «) ; 

 there are two on each side ; one anterior, on the front border of an 

 arch, a prezygapophysis ; one posterior, on the hind border, a post- 

 zygapophysis. From the under side of a centrum, in the middle 



