THE VKKTKHKAL CONSTITUTION OF THE SKELETON. 119 



In examining tho 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 funned by tho vertebral lamina; tho inferior 

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

 nated tin; 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 haemal arch (see Fig. 10). 



The haemal 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 haemal arches the following 

 parts : 



UAL ARCH. ll.KMAl. ABCH. 



1. \.-nral purapophysis = the posterior 



costal cupola. 

 1. NViirul iiirtu]H>i>hysis = the unU-rior 



.1 ClljMllil. 



:>. NYuial diajiopuysis = the summit of the 



transverse process. 



-I. NYurupophysis = the vertebral lamina. 

 .">. NYural spine = the summit of the 



spinous process. 



1. Haemal parapophysis = the tuberosity 



of the rib. 



2. Haemal metapophysis = the head of the 



rib. 



3. Haemal diapophysis = the rib proper. 



4. Hcumapophysis = the costal cartilage. 



5. Haemal spine = the corresponding 



sternal portion. 



The vertebrae 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 vertebrae ; or the 

 hii-mal arch is incomplete or null, as in the cervical or lumbar vertebrae ; 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 

 prot- 



Notwitstanding these differences and variations, or the transformations 

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

 not be included in the vertebral type. 



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

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

 thoraco-abdomiual region, the centrum, neural ffceh, and haemal arch are 

 readily perceived ; in the lumbar vertebrae, tho enormously developed trans- 

 verse process indicates the existence of an infravertebral arch. 



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

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

 to tho haemal arch, and should be considered as appendices of this arch, 

 analogous to the costal appendices of birds. 



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

 inferior lui-:nal arch is represented by tho osseous ring supporting the 

 anterior limbs tho scapulo-clavicular cincture. Tho limbs themselves arc 

 gpendioea of the cervical hrornal arch. 



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



and coccyx come to bo examined. Nevertheless, the composition of the 



I revealed when tho caudal vertebra) of certain ti < ially 



tlni-r of the pleuroneotkbe, in which the neural and ha-mal arches are -oin- 



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



' long time an insoluble question, or was sol veil in a eontrudictory 



