682 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
which they invest, and consequently extending’ further along the surface of the carti- 
laginous matrix. At fig. 37 g is a small tubular space, caused by a slight inflection 
of the lamellm, and at 37g' a large cavity has been produced in a similar way; in 
the latter instance, a narrow passage proceeds obliquely upwards and outwards, 
through the successive laminae, to the surface. This portion of the bone increases in 
size by the addition of new layers to its external surface, such layers being the 
result of a constantly progressing calcification of the fibrous periosteum with which 
it is covered ; w’hilst at each extremity of the membraniform osseous cylinder the ex- 
tending calcification takes place in the perichondrium, which appears to be merely an 
extension of the periosteum around the two cartilaginous extremities of the bone. 
No lacunae exist amongst the osseous layers. 
The original matrix of this bone has obviously been a small, cylindrical cartila- 
ginous rod, in which chondriform bone has been primarily developed as a superfieial 
ring midway between the two extremities ; a nearly connate layer of membraniform 
bone being formed by the calcification of a portion of the perichondrium. The two 
extremities have continued to be cartilaginous. The cells of the latter tissue have mul- 
tiplied in the ordinary manner at these extremities, where the cartilage has increased 
both in length and thickness. As this primary process has advanced, the membra- 
niform and chondriform bones have continued to increase, as already described. 
Whilst the two osseous tissues have been developed by independent processes of 
growth, they appear to maintain some peculiar relationship to one another; thus at 
the four points (fig. 37 e, e') they terminate together ; the external or intramembra- 
nous growth being but a very slight degree in advance of that formed within the 
cartilage. 
The above description will apply with the utmost exactness to the stylo-hyal bone 
of the same fish ; belonging to the same cylindrical class, it presents identical appear- 
ances. In the early state of the large cerato-hyal, precisely analogous details of 
structure and growth may be traced out ; and even in the matured fish, where the 
osseous elements are highly developed, the expanded cartilages of the two articular 
extremities are connected by means of a small narrow rod of the same tissue which 
passes through the centre of the bone. 
Before turning from the cylindrical to the flat bones, it is desirable to note the 
structure of one or two w'hich present an intermediate character. The epitympanic 
will best serve our' purpose ; since whilst in its depressed central portion it is closely 
allied to the flat bones, it is furnished with three long cylindrical processes or con- 
dyles. In its early state, a narrow layer of cartilage passes completely through the 
centre of this peculiar bone. In the adult fish this cartilage only appears externally 
at the extremities of the three condyles, which articulate respectively with the pre- 
frontal, mastoid and opercular bones. But conical prolongations of these terminal 
cartilages extend along the interiors of the condyles, in the direction of the centre of 
the bone ; and in a very young fisb these cartilages meet near the base of the external 
