ON THE VERTEBRATE SKELETON. 277 



the lepidosiren, the amphiuma or the apteryx, with the scarcely more simple 

 or less-developed appendage of the thoracic abdominal haemal arches (ribs) 

 of birds and fishes (figs. 15 and 17, «, a) ; and thus we are led to determine 

 its general homology, under its manifold forms of fin, fore-limb, wing, or arm, 

 as the diverging appendage of the haemal arch of the occipital vertebra. 



Thenatural and typical vertebral segment above-defined cannot bedetached, 

 in every fish, by the mere disjunction of sutures : in the lepidosiren, e. g. the 

 ossified part of the centrum has coalesced with that of the next segment in 

 advance and would require to be divided by the saw : the same coalescence 

 occurs in the human skull, and has led to the definition of the cranial bone, 

 called ' os spheno-occipitale*.' In osseous fishes, either by connation of 5 

 with 9, fig. 5, or by excessive development of bone in the notochordal capsule 

 extending forwards from the centrums, and producing 9, there results the long 

 bone (5, 9) continuing the series of vertebral centrums forwards, and corre- 

 sponding in position with two segments or arches above. On the hypothesis 

 that it represents the central elements of both those arches, it must be divided 

 artificially, in order to separate that segment of the cranium which next suc- 

 ceeds the occipital one. And, further, either by a similar coalescence of the 

 proximal elements of two haemal arches, or by the undue extension of such 

 element of one of the arches, interposing itself between the next arch and 

 the rest of the vertebra to which that arch belongs, it happens, that unless the 

 proximal element or elements in question be artificially divided, as at 28«, 28«, 

 fig. 5, two haemal arches (H 11 and H in) would be brought away, with the 

 neural arch detached by the separation of sutures and the division of the 

 bone 5, 9. If neither that bone, nor %%a were divided, but were, with the 

 bones in superior connection with them, separated from the bones anteriorly 

 articulated to them by suture, then we should have the group of bones, in- 

 cluded by the curved lines marked N 11, N in, Hn, H in in fig. 5. Two 

 vertebi-al segments are plainly indicated in this group by the distinct haemal 

 arches and their appendages, H n and H 111 ; but three pairs of bones, 16, 6 

 and to, fig. 5, appear to be in neurapophysial relation with the single and 

 symmetrical median bone 5, 9. If, however, what has been urged in the 

 chapter on ' Special Homology' (pp. 188-196) respecting the petrosal cha- 

 racter of 16 be a true interpretation of that bone, then we must eliminate it 

 from our present inquiry, inasmuch as being a partial ossification of a sense- 

 capsule (and nature herself removes them, as such, in most fishes), it apper- 

 tains to a category of bones (splanchno-skeleton), forming no part of the pro- 

 per neuro- or endo-skeleton, in which alone we seek for evidence of asegmental 

 disposition of parts corresponding with the segments of the nervous system. 



The bony petrosals (16) being removed, let us, then, with the view of ex- 

 amining the composition of the segment of the skull with which the occipi- 

 tal vertebra was articulated, saw across the bones 5, 9 and 2sa, and separate 

 the bones 6, 7, s from their sutural connections with those in front of them. 

 In thus obtaining the segment in question, the opponents to the vertebral 

 theory of the skull are entitled to assert that violence is done to nature by 

 the sections of the single bones above-cited ; the validity of which as an 

 objection to that theory will be afterwards inquired into. 



It is not, however, absolutely necessary to divide, the basal bone 5, 9 : in 

 many osseous fishes a symmetrical bone (fig. 5, 9') supports the parial bones 

 10, and stands in the relation of a centrum to them ; the neural arch or circle 

 of that segment would not, therefore, be broken by the removal with the 

 posterior segment of the whole of the bone 5, 9. If the corresponding 



* See Table I., Soemmerring. 



