ON THE VERTEBRATE SKELETON'. 265 



articular surface, and a thicker intermediate piece, as in all foetal mammals, 

 and throughout life in some cetaceans. 



With respect to function, the centrum forms the axis of the vertebral 

 column, and commonly the central bond of union of the peripheral elements 

 of the vertebra : as a general rule it supports, either immediately or through 

 the medium of the approximated or conjoined bases of the neurapophyses, 

 the neural axis (in the trunk called myelon, or spinal marrow, and its mem- 

 branes) ; the terminal centrums being usually deprived of this function by 

 the withdrawal of that axis from them in the course of its centripetal or con- 

 centrative movement. 



The neurapophyses are more constant as osseous or cartilaginous elements 

 of the vertebrae than the centrums; but they are absent, under both histolo- 

 gical conditions, at the end of the tail in most air-breathing vertebrates, where 

 the segments are reduced to their central elements. The neurapophyses lose 

 their primitive individuality by various kinds and degrees of confluence ; as 

 e. g. first, of the bases of each pair with their supporting centrum ; secondly, 

 of the apices of each pair with one another and with the neural spine, — the 

 lepidosiren affording a rare exception of the persistent individuality of this 

 element and of each neurapophysis throughout the trunk; thirdly, of two 

 or more neural arches with one another, as in the neck of some fishes, cetacea, 

 and armadillos, and in the sacrum of birds and mammals ; where they also 

 often coalesce with the pleurapophyses, as they do in the neck of most mam- 

 mals and birds. The neurapophyses rarely depart from the form of plates, 

 either broad or high, or both ; sometimes they are straight, sometimes arched, 

 sometimes bent ; sometimes by the inward extension of their bases, they form 

 together a bony ring above the centrum, excluding both that and the spine 

 from the neural canal. The neurapophyses may develope, as exogenous pro- 

 cesses, either diapophyses or zygapophyses, and the latter are sometimes 

 double from both the anterior and posterior borders of the plates ; as e. g. in 

 the vertebrae of Mugil, in some serpents, and in the lumbar vertebrae of some 

 mammals. The observed extent of variation of position of the neurapophyses 

 is from the upper surface of their own centrum to above the next intervertebral 

 space, so as to rest equally on two centrums ; or they may be uplifted bodily 

 from their centrum, and wedged or suspended between the two contiguous 

 neural arches, as e. g. in the atlas of ephippus and other deep-bodied fishes. 



Except in the cartilaginous neurapophyses of the sturgeon, I am not aware 

 of any instance of the subdivision of this element into two pieces, placed 

 vertically upon each other. Some plagiostomes show the principle of vegetative 

 repetition in two or three star-like centres of ossification, side by side, in the 

 primitive basis of the neurapophysis, but the second of the two cartilaginous 

 plates on each side of the neural canal, coextensive with the single centrum, 

 in most sharks, which second piece has the form of a wedge with the small 

 end directed down over the intervertebral space, seems to answer, as Prof. 

 Miiller has suggested, to the intercalary or interneural piece in bony fishes. 



The most constant functional relation of the neurapophysis is to protect 

 the spinal nerve in its exit from the spinal canal, either by a direct perfora- 

 tion of the neurapophysis (many fishes, and some mammals), by a notch in 

 the margin, or by the interspace between two neurapophyses. This function 

 alone is performed, in reference to the nervous system, at the posterior part 

 of the vertebral column in many animals, where the place of the shortened 

 myelon is occupied by the lengthened roots of the nerves : in the rest of the 

 trunk the neurapophyses protect also the neural axis. The original relation 

 of each neurapophysis to the segments of that axis is determined by the place 

 of connection of the perforating nerve with the shortened myelon. 



1846. t 



