ON THE ANATOMY OF FISHES. 
225 
by a cleft, and with or without the aid of those belonging to the fifth vertebra form 
a more or less complete investment to the dorsal and anterior walls of the air-bladder. 
The sixth is, as a rule, the first rib-bearing vertebra ; exceptionally, however, the first 
rib may be borne by the fifth, or even the seventh vertebra. 
In almost all cases, except where they are modified to form an “ elastic- spring- 
apparatus,” the transverse processes of the fourth vertebra, in addition to their 
characteristic relations to the air-bladder, form a more or less rigid support to the 
proximal elements of the pectoral girdle. 
Over a somewhat triangular area, on each side, between the exoccipital in front 
and the anterior margin of the arch of the complex vertebra behind, the wall of the 
neural canal is formed only by fibrous membrane, in which the claustrum, and the 
ascending processes of the scaphium and intercalarium when present, are imbedded. 
Of the four Weberian ossicles the claustrum has no physiological relations to the 
atrial cavities (atria sinus imparis of Weber), but merely strengthens the wall of the 
neural canal behind the exoccipital. Each scaphium has a spatulate process which 
fits into and completely closes the corresponding external atrial aperture, and at the 
same time forms the outer wall of the atrial cavity of its side, and usually also a 
rounded condylar process for articulation with the centrum of the first vertebra. 
The intercalarium is generally represented by an elongated or discoidal nodule 
imbedded in the stout ligament (“ interossicular ligament ”) connecting the scaphium 
with the tripus, and even if horizontal and ascending processes are present, the ossicle 
never articulates in the adult with the centrum of the second vertebra, to which, as 
a modified neural arch, it belongs. The tripus is always a tripartite ossicle with its 
posterior or crescentic process imbedded in the dorsal wall of the air-bladder ; the 
anterior process is directed forwards parallel to the long axes of the complex and 
first centra, and opposite the external atrial aperture of its side is connected by the 
transversely-disposed interossicular ligament with the convex outer surface of the 
spatulate process of the scaphium. The articular process usually articulates with the 
lateral surface of the third vertebral centrum, to which, as a modified transverse 
process or rib, the tripus probably belongs; very rarely {e.g., Aucheniptei'us) is the 
process directly continuous with the neural arch. 
The Weberian ossicles, or at all events the free portion of the tripus and the inter- 
calarium, are enclosed within a membranous saccus paravertebralis, the anterior wall 
of which is perforated by the interossicular ligament as the latter passes inwards 
from the tripus to its attachment to the scaphium. Unlike the Cyprinidm, the 
complete closure of the external atrial aperture by the spatulate process of the 
scaphium and the minute size of the hypoglossal foramen in the Siluridm comj^letely 
cut off all communication between the cavity of the saccus and the cranial cavity. 
With the exception, perhaps, of the Hypophthalmina and the flypostomatina, the 
relations of the anterior spinal nerves to the vertebral elements with which they are 
associated is remarkably constant. 
MDCCCXCm. — F5. 2 G 
