DEVELOPMENT OF THE VERTEBRAL COLUMN. 



627 



Fig. 310. 

 A. 



appeared to MM. Eathke * and Goodsir f, a similar disposition was ob- 

 servable : hence they conceived that the central axis of the nervous 

 system in this fish was reduced entirely to the parts representing the 

 spinal cord in other Yertebrata. Subsequent researches have, however, 

 shown that this is not strictly the case, but that, although there is no 

 cerebral enlargement corresponding to the encephalon of ordinary fishes, 

 the anterior extremity, inasmuch as there exist distinct olfactory J and 

 optic organs, must be regarded as essentially encephalic in its nature. 



(1685.) In tracing the modifications observable in the construction 

 of the vertebral column of fishes, we have a beautiful illustration of the 

 progressive advances of ossification in this the central portion of the 

 osseous system. The spine of the Lamprey, although at first sight ap- 

 parently entirely soft and cartila- 

 ginous, presents already, in the 

 arches which compose the spinal 

 canal, and in the soft cord that re- 

 presents the bodies of the vertebra, 

 slight indications of an incipient 

 division into distinct pieces : rings 

 of ossific matter are distinguish- 

 able, encircling at intervals the soft 

 spinal cartilage, upon which they 

 perceptibly encroach; so that, on 

 making a longitudinal section of 

 the cord, it offers the appearance 

 sketched in the adjoining figure 

 (fig. 310, A). In a more advanced 

 form of a fish's skeleton, as for 

 example in the Sturgeon, these 

 ossified rings are found to have enlarged considerably, and penetrate 

 still more deeply into the cartilaginous mass (fig. 310, B). As the bony 

 rings thus developed approximate the centre, it becomes more and more 

 evident that they represent the bodies of so many vertebra? ; but even in 

 the majority of fishes the central part remains permanently unossified, 

 so that a cartilaginous axis traverses the vertebral column from one end 

 to the other (fig. 310, c) ; and it is not unusual to find the central aper- 

 ture perfectly obliterated, as delineated in the fourth sketch (D). 



(1686.) Fishes, being continually resident in an element nearly of the 

 same specific gravity as their own bodies, require little firmness or 

 solidity in the construction of their spinal column : a free and unfet- 

 tered power of flexion in certain directions, so as to permit an ample 



* " Bemerkung iiber don Bau des Amphioxus lanceolatus" Monatsbericbte der 

 Akad. der Wissenschaften, 1841. 



t Transactions of the Royal Society of Edinburgh, vol. xv. 

 Vide Kolliker, Muller's Archiv, 1843. 



Development of vertebral column. 



