ON THE ANATOMY OF FISHES. 
81 
doi’sally is continued forwards as a roof to the atria, as far as the hinder edge of the 
roof the cavum sinus imparis, where it apparently becomes continuous with the very 
thin layer of dura mater investing the inner surface of the cranial bones. In the 
anterior wall is the median foramen, through which the two atrial cavities freely 
communicate with the cavum sinus imparis, but on each side of this the anterior wall 
is formed by the concave posterior edge of the root of the exoccipital. The floor of 
the atria is formed by the basioccipital, while their outer walls coincide with two 
deep semicircular notches in the posterior margins of the exoccipitals. The notch 
in each exoccipital, with the adjacent dorso-lateral margin of the basioccipital, form 
the anterior, superior, and inferior boundaries of a lateral orifice (at. a.), the “ apertura 
externa atrii ” of Weber — by which each atrium would communicate with the 
exterior of the skull were it not that the aperture is completely closed by the spoon- 
shaped extremity of the “stapes” of its side (fig. 9, sc.s., also figs. 4 and 5, at. a.). 
The posterior boundary of each external atrial aperture is formed by the same 
thick mass of fibrous tissue that forms the posterior atrial wall. The inwardly 
curved inferior extremities of the two claustra lie immediately over the external 
atrial apertures, and may help to strengthen their dorsal lips, and possibly also 
contribute to the support of the atrial roof. The two atrial cavities are in open 
communication with each other and with the cavum sinus imparis anteriorly ; in 
fact, if it were not for a thick vertical ridge of fibrous tissue projecting inwards 
from the median line of their posterior wall (fig. 9), they might be regarded as 
constituting a single transversely-disposed chamber. Essentially, the two atria may 
be considered as forming a bilobed posterior prolongation of the cavum sinus imparis 
excavated in the substance of a thick mass of fibrous tissue which forms the floor 
of the neural canal over the body of the first vertebra and the contiguous dorsal 
surface of the basioccipital. The fibrous tissue forming the roof and posterior wall 
of the atrial cavities is continuous laterally with the fibrous walls of the neural 
canal between the arch of the third vertebra behind, and the exoccipitals in 
front. 
In his account of these structures in Amiurus, Ramsay Weight (43) describes an 
anteriorly bilobed median excavation in the substance of the tissue overlying the 
medulla oblongata which is said to communicate on each side with the atrial cavities. 
This excavation or receptaculum dorsale is described as being “ filled with the same 
fluid contents as the atria and cavum.” A precisely similar receptaculum exists 
in Macrones, and communicates with the atrial cavities by an aperture, which, on 
each side, perforates their fibrous roof near the superior border of the sjiatulate 
portion of the “stapes.” 
From the position and relations of the various factors of the membranous labyrinth 
and of the bony chambers or recesses in which they He, it will follow that the 
perilymph will not only surround the utriculiis and semicircular canals, but also 
extend into the fovem sacculi and cavum sinus imparis, and surround their 
MDCCCXCIII. — B. 
.M 
