THEORY OF TEE YERTEBRAL CONSTITUTION OF THE SKELETON. 169 



abdominal region, the centrum, neural arch, and hsemal arch are readily per- 

 ceived ; for in the lumbar vertebrae, the enormously developed transverse process 

 indicates the existence of an intra-vertebral arch. 



In the sacral region, the bony girdle of the pelvis represents the hsemal arch. 

 The posterior limbs, articulating with the bones of the pelvis, also 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 haemal arch is represented by the osseous ring supporting the anterior 

 hmbs — the scapulo-clavicular girdle. The limbs themselves are appendices of 

 the cervical haemal arch. 



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

 coccyx — 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 hsemal arches are complete, are examined 

 (Fig. 113). But the vertebral constitution of the head remained for a long time 

 an insoluble question, or it was solved in a contradictory manner by the 

 naturalists who attempted it. Some admitted a single cranial vertebra ; others 

 included three or four ; while others, again, found six or seven. 



These difficulties and contradictory results may be understood, when it is 

 borne in mind what profound modifications the vertebra must have undergone 

 to constitute the bones of the head. 



At present the problem appears solved. The head is composed of four 

 vertebrae, in which are found the various parts enumerated in the description of 

 the typical vertebra. 



In the four classes of vertebrata, the head is constantly formed of four 

 vertebrse, which are determined as follows, according to Lavocat : — 



