THEORY OF THE VERTEBRAL CONSTITUTION OF THE SKELETON. 167 



CHAPTER IV. 



THEORY OF THE VERTEBRAL CONSTITUTION OF THE 

 SKELETON. 



In the series of vertebrated animals, the bony pieces of the trunk bearing the 

 name of vertebrse are those which offer the highest degree of fixity, and to which 

 the existence or the arrangement of the others appears to be subordinate. This 

 feature in organization, recognized by E. Geoffroy Saint-Hilaire and Professor 

 Owen, has caused these authorities to assert that the type of construction of 

 vertebrated animals is the vertebra. 



After E. Geoffroy Saint-Hilaire and Professor Owen, several German, English, 

 and French anatomists have studied the vertebral composition of the skeleton ; 

 and among the works published in France on this subject, must be specially 

 noticed those of Lavocat. In principle, all the writers have arrived at the same 

 conclusions, and only differ in some few details. 



It is certain that the base of the vertebral column is formed by a series of 

 bony segments. Each of these segments is called an osteodesm, and each osteo- 

 desm represents the bodi/ or centrum of a vertebra. 



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 laminae ; the inferior by the ribs, their 

 cartilages, and a portion of the sternum. The first is designated the neural arch, 

 as it furnishes a protective case for the nervous centres ; and the second, which 

 more particularly protects the vascular system, is called the hcemal arch (see 

 Figs. 112, 113). 



The hfemal 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 htemal arches the following parts : — 



H^MAL ARCH. 



1. Haemal parapophysis = the tuberosity of 



the rib. 



2. Haemal metapophysis = the head of the 



rib. 



3. Haemal diapophysis = the rib proper. 



4. Hsemapophysis = the costal cartilage. 



5. Haemal spine = the corresponding sternal 



portion. 



NEURAL ARCH. 



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. 



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 hsmal arch is incom- 

 plete or null, as in the cervical or lumbar vertebrae ; or, lastly, the arches are 

 often unequal ; though this inequality is of no importance, since their size is in 

 relation to the volume of the parts they should protect. 



Notwithstanding these differences and variations, or the transformations 



