674 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
of PuRKiNJE. The former exhibit none of the beautiful radiating lacunae which are 
so characteristic of the latter objects. It is true, we have frequently a small tubular 
canal connecting tvvm such cavities together, but these are very different from the 
beautiful stellate objects seen in the bones of an eel or the scales of a Lepidosteus. 
The fact is, that though analogous they are not homologous. They differ in their 
structure, and still more in their origin ; and the neglect of these differences has con- 
tributed, in no small degree, to produce the amount of obscurity and confusion that 
has hitherto invested the history both of the corpuscles of Purkinje and the genesis 
of bone, an obscurity which Dr. Sharpey was the first to clear away. 
Turning from the flat Plagiostomes to the Sharks, we find that their osseous struc- 
tures are essentially the same as amongst the Rays, though presenting numerous 
minor modifications, both in their modes of development and in the forms which 
their osseous centra ultimately assume. 
The common Picked Dogfish {Spinax acanthias^ Cuv.) presents one of the simplest 
types, and is one which can be studied with facility owing to the readiness with which 
examples can be obtained in different stages of their growth. The small plates with 
which the surfaces of the cartilages are covered, are not above one-fifth or one-sixth 
the size of those seen in the Ray, and are consequently more numerous in the same 
area ; in other respects they closely resemble them. Mr. Millar has already pointed 
out the fact that the ossified centrum of the vertebra of the Dogfish is shaped like a 
hour-glass*. It is in fact an osseous cylinder, constricted in its centre and sur- 
rounding the canal of the chorda. If a vertebra be taken from a very young Dog-fish, 
and a section of it be prepared in the direction of that of the Ray (fig. 28), viz. 
vertically, midway between the two concave extremities, we shall perceive that the 
canal of the chorda dorsalis is surrounded by a very narrow ring of bone. Between 
the inner surface of this ring and the fibrous membrane lining the canal, there exists 
a considerable interval which is occupied by cartilage, in which the cells are uniformly 
diffused, excepting in the immediate vicinity of the bone, where they exhibit a slight 
disposition to assume a concentric arrangement. External to the osseous ring is the 
great bulk of the cartilage forming the vertebra. In that portion of it which sur- 
rounds the bone, the cells are arranged with great regularity in lines running from 
the ring towards the periphery. In the more external portions this radiated disposi- 
tion is less obvious. Nearly the whole of the peripheral portions are invested by 
a thin osseous film, consisting of the small plates already spoken of. Similar ones 
line the greater portion of the neural canal, but are absent from its upper part, both 
externally and internally. These two series of external and internal plates are con- 
nected by means of the canals which allow of the exit of the spinal nerves ; they also 
being lined with similar plates. The floor of the neural canal is occupied by one 
plate of much larger size than the rest, whilst external to it, but within the canal, 
is a small mass of cartilage, so that this bone is invested with cartilage on both its 
* Footprints of the Creator, p. 43, fig. 8 a. 
