12 



piece, (neural spine, or neuracantha), which may he suhdivided in 

 the median plane. 



The inferior or haemal arch is also constituted when most developed 

 (tail of the lepidosiren) by three elements ; the two lateral (haemal 

 laminae or angiopomata) and the azygos inferior one (angiacantha 

 or haemal spine), which is never subdi\-ided. This arch is most 

 generally present in the caudal region, disappears in the trmik, and 

 reappears in the cervix. In man it only exists at the junction of the 

 occipital and atlantal vertebrae, forming the so-called ' body of the 

 atlas,' which is regarded by me as the haemal arch of the third 

 cranial vertebra displaced backwards to the intervertebral interspace, 

 as in the caudal region. 



The visceral arch, which is also inferior but external to the last, 

 may be regarded as composed of an azygos inferior and two lateral 

 elements. The former is the sternal segment and may be subdivided 

 mesially. Each lateral piece is also resolvable generally into an upper 

 segment (vertebral rib or pleura); and a lower one usually cartila- 

 ginous (sternal rib or hypopleura), which may be subdivided into two 

 or three pieces (three in Plesiosaurus) . 



The segmentation of the vertebrae is partly due to the laws which 

 preside over their genesis, and partly determined by teleological causes. 



Several of the elements unite to form the vertebra of the anthro- 

 potomist ; thus the constituents of the neural arch coalesce with the 

 centrum in the dorsal vertebrae ; while in those of the cervical, lum- 

 bar and sacral regions, the abortive pleural complements also are an- 

 chylosed to the elements just mentioned. 



In fishes, the lower part of the vertebral body is formed by the ex- 

 panded bases of the angiopomata, which meet those of the neuropo- 

 mata and enclose the proper centrum ; but in the higher vertebrata 

 the greater development of the centrum excludes the angiopomata 

 from any share in the body, and displaces them backwards to the in- 

 tervertebral interspace next in succession. 



The coexistence of the visceral and haemal arches is seen in fishes, 

 in the cervical region of many lacertae, and in the tails of the lizards 

 and crocodiles, &c. 



Therefore the one is not convertible into the other, as has been 

 supposed by Professor Owen, who regards the sternum and sternal 

 ribs in the thorax as the equivalents of the angiacantha and angiopo- 

 mata, the latter being dislocated from their normal attachment to the 

 centrum and suspended to the extremities of the corresponding pleural 

 elements constituting the sternal ribs, while the former is expanded 

 and sometimes divided mesially to form the sternum. 



I am therefore compelled to suggest a new nomenclature of the 

 elements of a typical vertebra more conformable to nature than 

 that employed by Professor Owen, who has used the same term for 

 several distinct objects, and given two different appellations to the one 

 and the same element. 



My view of the typical vertebra is that which has been adopted 

 by the distinguished German anatomists Miiller, Rathke, &c. 



