688 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
seen in all the other portions of the endoskeleton, the arrangement of the various 
structural elements being modified in accordance with the peculiar form of the bone 
into which they enter. On the other hand, they separate it wholly from scale struc- 
tures, in connection with which cartilage and chondriform bone are alike unknown. 
It is scarcely necessary to add, that no visible cells enter into the composition of the 
membraniform lamellse of the opercular of the Pike, any more than into the other 
analogous osseous structures which have passed under review. 
The only remaining element of the skeleton of the Pike requiring a special notice 
is the vertebra, the growth of which is highly interesting : whilst the principles which 
have regulated the construction of the other bones are still adhered to, they have 
been subjected to such modifications as were rendered necessary by the peculiar form 
of the bone, and the functions which it had to fulfil. 
Fig. 41 represents a transverse section of one of these vertebrae, made at right 
angles to the spinal axis and midway between its two terminal cones. In the centre 
is a small cavity, 41 a, through which the remains of the chorda dorsalis have been 
prolonged, and which is surrounded by a very thin ring of membraniform bone ; from 
this ring there radiate eight conical segments having different structures ; four of 
these are osseous, alternating with the other four which are cartilaginous. One of 
the osseous segments (41 h) proceeds upwards, to form the floor of the neural canal. 
Two others, somewhat larger than the last, 41 c, c, pass upwards and outwards, 
whilst the remaining one, occupying above one-fourth of the area of the section, 
passes both downwards and outwards, constituting the most substantial part of the 
centrum. When the vertebra is entire, these osseous segments appear as longitudinal 
plates, passing from the one terminal cone to the other. 
Each of these segments is composed of large and irregular cancelli of membrani- 
form bone, the walls of which have a laminated structure ; but towards the periphery 
the laminse lose their irregular distribution, and form a series of small radiating 
marginal plates, 41 e, between which are numerous open spaces, allowing a free com- 
munication to exist between the exterior and interior of the bone. These plates do 
not exist along the middle of each segment, which is occupied by a deep excavation, 
especially large in the inferior segment, 41 d'. These excavations correspond with 
longitudinal grooves which run along the surface of the radiating plates. 
The lamellse entering into the composition of the osseous laminae, or walls of the 
cancelli, usually follow the same direction as the exteriors of these laminse. But in 
the marginal plates (41 e) they appear in the form of investing cones, arranged parallel 
to the external outline of each plate, and being evidently the result of successive 
growths added to its exterior. The space intervening between these osseous seg- 
ments are occupied by four corresponding ones of cartilage, 41 /, g, but which are 
of more uniform size. Of these, the peripheral portions of the two uppermost corre- 
spond with the bases of the neurapophyses, 41 A, whilst the lower ones bear the same 
relation to the parapophyses, 41 i. 
