TBE VERTEBRAL CONSTITUTION OF THE SKELETON. 119 



In examining the dorsal region, it is evident that to the body or centrum 

 of a vertebra are added two complete osseous arches, a superior and an 

 inferior. The superior arch is formed by the vertebral lamina ; the inferior 

 by the ribs, the cartilages, and a portion of the sternum. The first is desig- 

 nated tl^e neural arch, because it furnishes a protecting sheath for the 

 nervous centres; and the second, which more particularly protects the 

 vascular system, is called the hmmal arch (see Fig. 10). 



The hsemal arch may have prolongations or appendices more or less 

 developed, and comparable to the apophysary prolongations of the ribs in 

 birds and some fishes. 



Such is the general composition of a typical vertebra; but there are 

 also to be distinguished in the neural and heemal arches the following 

 parts : — 



NECEAL AKCH. 



1. Neural parapophysis = the posterior 



costal cupola. 



2. Neural metapophysis = the anterior 



costal cupola. 



3. Neural diapophysis = the summit of the 



transverse process. 



4. Neurapophysis = the vertebral lamina. 



5. Neural spine = the summit of the 



spinous process. 



HiEMAL AEOH. 



1. Hsemal parapophysis = the tuberosity 



of the rib. 



2. Haemal metapophysis = the head of the 



rib. 



3. Hsemal diapophysis = the rib proper. 



4. Heumapophysis = the costal cartilage. 



5. Haemal spine = the corresponding 



steinal portion. 



The vertebrte sometimes depart more or less from the model just 

 described. They may vary not only from one species to another, but also 

 in the same animal, and even in the same region. Thus the neural arch 

 may be absent, as has been observed in certain coccygeal vertehrEe ; or the 

 hsemal arch is incomplete or null, as in the cervical or lumbar vertebra ;" or, 

 lastly, the arches are often unequal ; though this inequality is of no import- 

 ance, since their size is in relation to the volume of the parts they should 

 protect. 



Notwitstanding these differences and variations, or the transforfiaations 

 experienced by certain parts, there is not a bone in the skeleton which can- 

 not be included in the vertebral type. 



The vertebra being admitted as the type of construction of the skeleton, 

 it is easy to find it in all the regions of the bony framework. In the 

 thoraco-abdominal region, the centrum, neural arch, and hsemal arch are 

 readily perceived ; in the lumbar vertebras, the enormously developed trans- 

 verse process indicates the existence of an infravertebral arch. 



In the sacral region, the bony girdle of the pelvis represents the haemal 

 arch. The posterior limbs, articulating with the bones of the pelvis, belong 

 to the hsemal arch, and should be considered as appendices of this arch, 

 analogous to the costal appendices of birds. 



The cervical region may be compared to the sacral region ; as in it the 

 inferior hasmal arch is represented by the osseous ring supporting the 

 anterior limbs — the scapulo-clavicular cincture. The limbs themselves are 

 appandices of the cervical haemal arch. 



Difficulties begin to appear when the extremities of the trunk — the head 



and coccvx come to be examined. Nevertheless, the composition of the 



coccyx is" revealed when the caudal vertebrae of certain fishes, especially 

 those of the pleuronectidae, in which the neural and h^mal arches are com 

 Dlete are examined. But the vertebral constitution of the head remained 

 for a' long time an insoluble question, or was solved in a contradictory 



