ANATOMY OE VERTEBRATES. 29 



smaller as they approach the end of the vertebral column ; one 

 element or process after another is removed, until the vertebra 

 is reduced to its centrum, as in the subjoined diagram, fig. 21, 

 of the archetype vertebrate skeleton. 



15. Archetype skeleton. In this scheme, which gives a side 

 view of the series of segments or ( vertebra? ' of 

 which the skeleton is composed, the extreme 

 ones are the seat of those modifications, which, 

 according to their kind and degree, impress 

 class-characters upon the type. 



The four anterior neurapophyses, 14, 10, 



n 6, 2, give issue to the nerves, the terminal ^^ v - >& *r pl 

 modifications of which constitute the organs 

 of special sense. That of smell, 4, 19. is 



1 . Cervical segment or 



situated in advance of its proper (nasal) seg- vertebra 



meiit, which becomes variously modified to enclose and protect 

 it. The organ of sight, lodged in a cavity or 'orbit' be- 

 tween its own (the frontal) and .the nasal segment, is here 

 drawn above that interspace. The nerve of taste perforates 

 the neurapophysis of the third segment, 6, or passes by 

 a notch between this and the frontal segment, to expand in the 

 sense-organ, or ' tongue,' which is supported by the haemal spine, 

 41, 42, of its own (parietal) segment. The fourth is the organ 

 of hearing, 16, indicated above the interspace between the neura- 

 pophysis of its own (occipital) and that of the antecedent (parietal) 

 vertebra, in which it is always lodged ; the surrounding vertebral 

 elements being modified to form the cavity for its reception, which 

 is called f otocrane.' 



The jaws are the modified haemal arches of the first two seg- 

 ments. The mouth opens at the interspace between these haemal 

 arches ; the position of the vent varies (in fishes), but always 

 opens behind the pelvic arch, s, 62, 63, p, when this is ossified. 



Outlines of the chief developements of the dermoskeleton, in 

 different vertebrates, which are usually more or less ossified, are 

 added to the neuroskeletal archetype ; as, e. g. the median horn 

 supported by the nasal spine, is, in the rhinoceros ; the pair of 

 lateral horns developed from the frontal spine, n, in most rumi- 

 nants ; the median folds, D i, D 11, above the neural spines, one or 

 more in number, constituting the ' dorsal ' fin or fins in fishes and 

 cetaceans, and the dorsal hump or humps in the buffaloes and 

 camels ; similar folds are sometimes developed at the end of the 

 tail, forming a ' caudal ' fin, c, and beneath the haemal spines^ 

 constituting the ( anal ' fin or fins, A, of fishes. 



