VEKTEBRAL COLUMN OF FISHES. 53 



Returning, tlien, to what may be called the high road of vertebral 

 development, we find in the Sturgeons (^Stiirio, Polyodon), that the 

 inner layer of the fibrous capsule of the gelatinous ' chorda' has in- 

 creased in thickness, and assumed the texture of tough hyaline car- 

 tilage. In the outer layer are developed distinct, firm, and opaque 

 cartilages, the neurapophyses, which, in the young sturgeon (_^^. 12.), 

 are two superimposed pieces on each side, the basal portion bounding 

 the neural canal, the apical portion the parallel canal filled by fibrous 

 elastic ligament and adipose tissue*; above this is the single car- 

 tilaginous neural spine. The parapophyses are now distinctly de- 

 veloped, and joined together by a continuous expanded base, forming 

 an inverted arch beneath the ' chorda ' for the vascular trunks, even in 



N"rurnl spine. 



Fibro-adipose , A' 



canal. — \'V\\ — V Neurapoph;, sis. 



Neural canal._^_^'-^ Iiiterneural cartilage. 

 Gelatinous chorda.. 



CZI3/^~--v^/t:~'Ov — '^j^^^IIDl'liurapo)>liysis. 

 Inner layerAxI "v^-j 1/ P.!rapo|ili.vsis. 

 of fibrous capsule ^^O^— V^'" '""'''''=^'"'^' cartilage. 

 as \i\ aline cartilage. ^^T^^^J^ 



^^=i^>. Hscmal canal. 



Abdominal vertebra, Smrgeon. 



the abdomen. Short and simple pleurapophyses are articulated by 

 ligament to the ends of the laterally projecting parapophyses in the 

 first twelve or twenty abdominal vertebrae ; the parapophyses them- 

 selves gradually disappear, or bend down to form ha3mal arches in the 

 tail, at the end of which we find haemal cartilaginous spines cor- 

 responding to the neural spines above. The first five or six neural 

 arches are confluent with each other in the stui-geon, and, with the 

 parapophyses, enclose the fore part of the ' chorda ' in a firm, con- 

 tinuous, cartilaginous sheath, perforated for the exit of the nerves. 

 The tapering anterior end of the ' chorda ' is continued forwards 

 into the basal elements of the cranial vertebrae. 



Vegetative repetition of perivertebral parts not only manifests 

 itself in the double neurapophysis on each side, but in a small 

 accessary (interneural) cartilage, at the fore and back part of the 

 base of the neurapophysis ; and by a similar (interha^mal) one at the 

 fore and back part of most of the parapophyses. The peripheral 

 cartilages are more feebly developed in the Polyodo7i. f 



* I long ago pointed out, in a preparation of Hunter's (No. 2.T4.), the " spaci; above 

 the canal of the spinal chord fornied by tiie divarication of the cartilaginous ))ieces 

 which constitute the support of the spinous processes of the vertebras. 'I'liis is filled 

 by lihro-cartilaginoiis subst;uice, connecting the processes in (piestion." (xx. vol. \.) 



t Ciivier, IMeuioires ilu Rluseuni, torn. i. 1815, p. l;U). 



E 3 



