ON THE ANATOMY OF FISHES. 
115 
lateral surface of the anterior third of the complex centrum obliquely outwards and 
upwards to blend with the ventral surface of the root of the first of the modified 
transverse processes. Each lamina is situated ventrad to the corresponding posterior 
cardinal vein and indeed forms the inferior lip of the anterior opening of the bony 
canal through which the vein passes. At the ventral and anterior extremity of each 
dorsal lamina, and but partially confluent therewith, there is a large crescent-shaped 
radial nodule (r.n.). The lateral surface of the body of the fifth vertebra is traversed 
by a stout ridge which, commencing near the anterior end of the centrum, extends 
obliquely backwards and upwards, and eventually bends directly outwards on to the 
ventral surface of the transverse process (fig. 32). This ridge receives the dorsal 
insertion of the primary transverse septum of the air-bladder. 
In the skull of Platystoma the neural plates of the exoccipitals are undeveloped, 
the lateral boundaries of the transversely oval foramen magnum being formed by the 
concave inner margins of the posterior plates. A stout process of bone projects 
directly backwards from each posterior plate, and from this process one of the two 
great compressor muscles of the air-bladder takes its origin. The posterior wall of 
the atria sinus imparis is strengthened in the median line by a small vertical column 
of bone wdiich extends upwards from the dorsal edge of the posterior face of the basi- 
occipital and nearly meets the hinder margin of the roof of the cavum sinus imparls. 
The air-bladder is well developed and extends backwards as far as the origin of the 
eighth pair of ribs (fig. 33, a.h.). In shape it is somewhat ovoid, being but slightly 
broader in front than behind. The lateral portions of its anterior wall are partially 
supported by the anterior divisions of the first pair of transverse processes (fig. 34, 
tp}, a.) ; posteriorly to this the outer walls are at first in contact for a limited extent 
with the lateral cutaneous areas, but subsequently separate from the skin and the 
lateral musculature of the body wall as they contract to enclose the relatively 
narrower posterior section of the bladder. A remarkably strong muscle has its 
insertion into nearly the whole extent of each half of the ventral wall of the anterior 
thu’d of the bladder, and its fibres, gradually converging as they are traced forwards, 
curve round the anterior wall and pass dorsally between the inferior limb of the post- 
temporal and the transverse process of the fourth vertebra to their origin on the 
posterior plate of the exoccipital a little to the outer side of the foramen magnum 
(fig. 33, cp., m.). As the contraction of these muscles must necessarily lead to the 
forcible compression of the anterior chamber of the bladder, and possibly to the expul- 
sion of its gaseous contents, we shall in futm'e refer to them as the “ compressor 
muscles.” A much smaller and hitherto undescribed muscle {t.t.) occupies the recess 
on each side of the centrum of the first vertebra, between the latter and the corre- 
sponding compressor muscle. This muscle also arises from the posterior plate of the 
exoccipital, but its fibres pass directly backwards, and are prolonged into a long and 
slender tendon, which is ultimately inserted into the anterior wall of the bladder in 
close proximity to the attachments of the latter to each radial nodule (figs. 33, 34, t.t.). 
Q 2 
