ANATOMY OE VERTEBRATES. 29 



smuUcr as they apjiroach the end of the vertebral column ; one 

 clement or process after another is removed, until the vertebra 

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

 of the archetype vertebrate skeleton. 



§ 15. Archeti/pe skeleton. — In this scheme, which gives a side 

 view of the series of segments or ' vertebraj ' of 20 



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, io, 

 n 6, 2, give issue to the nerves, the terminal 

 modifications of which constitute the organs 

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

 situated in advance of its proper (nasal) seg- 

 ment, which becomes variously modified to enclose and protect 

 it. The organ of sight, lodged in a cavity or 'orbit' l)e- 

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

 drawn above that interspace. The nerve of taste perforates 

 the uenrapophj'sis 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 hannal 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 ' otocrane.' 



The jaws are the modified ha3mal arches of the first two seg- 

 ments. The month opens at the interspace between these liKinal 

 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 

 difterent vei'tebrates, which are usually more or less ossified, are 

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

 supported by the nasal spine, 15, in the rhinoceros; the pair of 

 lateral horns developed from the frontal spine, 11, in most rumi- 

 nants ; the median folds, D 1, D u, 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 bufialoes and 

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

 tail, forming a ' caudal ' fin, C, and beneath the hromal spines, 

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



