ON THE VERTEBRATE SKELETON. 2/5 



those next in advance, we have the circle, or the base hone (i)and arch 

 (2, 3, 4), represented in figure 1, and we also bring away, articulated therewith, 

 an inferior or inverted arch with its appendages, represented in profile outline 

 in fig. 5, 50-57 : the arrow indicating the course of convergence, and its head 

 the point of union, of the two flanks or crura, forming the closing point or 

 crown of such inverted arch. 



We have thus removed a segment of the skull, and with as little or even 

 less violence or disturbance to the other bones, than must have been used in 

 detaching a similar segment from the thorax or pelvis of a land-animal. If 

 we compare this cranial segment with the typical vertebra fig. 14, we recog- 

 nise in the single median bone (1, fig. 1) the centrum, by its relative position 

 and its articular surface for the atlas, which retains, moreover, the concave 

 form characteristic of the vertebrae in the piscine class : in the pair of bones 

 (a, 2), which articulate with the upper surface of the centrum, protect the 

 sides of the epencephalon, and are perforated by the ' nervi vagi,' we have the 

 neii? -apophyses : in the single symmetrical bone (3) which completes the 

 arch, and terminates in a crest for the attachment of the uppermost or dorsal 

 portions of the vertebral muscles continued from the trunk, we have the neural 

 spine : and in the pair of bones (4, 4), wedged between this spine and the 

 neurapophyses, which give attachment to the inferior arch of the segment 

 (fig. 5, H i), and terminate in a free crest or spine for the attachment of the 

 upper and lateral portions of the vertebral muscles, we have the parapo- 

 physes ; for whose elevated position we have been prepared by their gradual 

 ascent in the anterior vertebrae of the trunk. The rest of this natural segment 

 has undergone the same kind of modification as the thoracic vertebrae present 

 in higher animals (fig. 15), and which consists in the great expansion of the 

 haemal arch, the removal of the hcemapophyses (fig. 5, 52) from the centrum 

 (ib. i),and the interposition of elongated and defected pleurapophyses (50, 51): 

 finally, the great inverted arch, so formed, encompasses, supports and protects 

 the heart, or centre of the haemal axis. The elements of this arch are open 

 to two interpretations according to the type of figure 15 : either 50 may be 

 pi, 51, h and si lis; or 50 and 51 may be a divided (teleologically compound) 

 pleurapophysis, and 52 an unusually developed haemapophysis : and this latter 

 conclusion is more agreeable with the character of the vertebral segments of 

 the trunk in fishes, in which the haemal spines are absent, the haemapophyses, 

 when ossified, long and sometimes joined together at their lower ends, as e. g. 

 in the first trunk- vertebra of Argyreiosus vomer, and the pleurapophyses some- 

 times, as e. g. in the sturgeon, composed of two or more pieces, set end to 

 end. The condition of the pleurapophysis of the pelvic arch in the meno- 

 pome (fig. 28, 62, pi), which sustains a radiated appendage (ib. A) of the 

 haemal arch of the occipital vertebra, indicates the true character of the 

 pleurapophysis : and the modifications of this arch in the higher classes will 

 be found to establish the accuracy of the general homology of the bone 52, 

 with the haemapophysial element, since the lower extremities of 52 are actu- 

 ally drawn apart and articulated to a haemal spine, which completes the arch 

 below in reptiles and birds (fig. 22, H s). 



Even should there be error in assuming the subdivision of the pleurapo- 

 physes and the absence of the haemal spine, in the particular determination of 

 the constituent elements of the arch in question, yet the alternative is still 

 within the recognised limits of the vertebral modifications of the trunk ; and 

 the want of unquestionable proof of the precise elements forms no valid ob- 

 jection to its general homology as a haemal vertebral arch, expanded and modi- 

 fied after one or other of the types of those which, in the thorax of the air- 

 breathing vertebrates, encompass and protect the more backwardly placed 



