VERTEBRAL COLUMN OF FISHES. 



55 



Vertical transverse section of 

 centrum of Selachc maxima. 



lons-itudinal 



tebral biconieal spaces.* The rest of the centrum is strengthened by 

 a beautiful arrangement of osseous phites, witli intervening hiyers 

 of cartilage {fig. 13.). Four sub-compressed conical cavities ex- 

 tend, two from the bases of the neura- 

 popliyses (w, ri), and two from those of the 

 parapophyses {p, p) towards the centre of 

 the vertebral body, contracting as they pe- 

 netrate it. These cavities always remain 

 filled by a clear cartilage : the central two- 

 thirds of the vertebral body contain con- 

 centric and minutely perforated rings or 

 cylinders of bone, interrupted by the four 

 depressions : the peripheral third contains 

 bony laminas, which radiate, perpendicularly to the 

 plane of the outermost cylinder, toward the periphery of the ver- 

 tebra: these outer laminae lie, therefore, parallel with the axis of 

 the vertebra, and the intervening fissures, like those between the 

 concentric cylinders within, are filled by clear cartilage, which shrinks 

 and leaves them open in the dry vertebra. There is a transition from 

 the cylindrical to the longitudinal lamellar structure ; the outer 

 cylinder being broken up, and sending out processes which join the 

 irregular inner edges of the outer lamellce. 



There are few examples in the animal economy in which the 

 smallest possible quantity of earthy matter is arranged according to 

 such beautiful and clearly manifested mechanical principles, for 

 affording the greatest amount of strength, and that degree of resistance 

 which the necessarily light, semi-ossified vertebra of a gigantic Shark, 

 maintaining itself near the surface by mvxscular exertion, without 

 help from a swim-bladder, must have to sustain during the vigorous 

 inflexions of the vertebral column, producing the violent compressions 

 of their interposed elastic balls. 



I have been induced to enter into the details of the condition of 

 the vertebrae of the Selache, both on account of the large scale on which 

 the beautiful structure is shown, and because of the meagre notice of it 

 in Home's " Anatomy of the Basking Shark {Srpialusmaximus^)" of 

 which John Miiller justly complains. Mliller's inference that the 

 vertebrae of Selache resemble those of Lamna is correct : but in 

 Lamna cornuhica the outer longitudinal plates are fewer, and are 

 bent so as to intercept long elliptical spaces filled with cartilage. 



* Mr. Clift fouiul, on picrciii.if the capsule with a knife, tliat the contained fluiil 

 was spirted out to a considerahle distance, by the contraction or recoil of the tensely 

 filled elastic hag. Sec Prep. Nos. 2;n a. and 237 b. and xx. vol. i. 1832. 



t Phil. Trans. 1809, p. 177. 



E 4 



