86 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
Scapldum. mm. 
Length of ascending process, including the condylar process .... 3 
Total length of the spatulate process, including its root 2‘5 
Length of the stapulate process, not including its contracted root, but 
only the eoncavo-convex portion which closes the external atrial 
apertare 2 
Maximum width of the spatulate process 1 
hitcrossicular Ligament. 
Length 1 '2 
Intercalarium. 
Length of the ascending process 4 
Length of the horizontal process 3 
Tripus. 
Total length not including curvature of the crescentic process .... 8 
Length of anterior process 4 
Length of the crescentic process, not including its curvature 4 
Length of the articular process 2 
Claustrum. 
Length • 3o 
The Sacciis Pamvertebralis . — The free portion of the tripus, that is to say, the 
articular and anterior processes, with the posterior half of the interossicular ligament, 
and the horizontal process of the intercalarium, are situated within the cavity of a 
thin-walled fibrous sac — the “saccus paravertebralis ” of Weber. The sac extends 
from near the external atrial aperture of its side along the lateral surfaces of the 
centrum of the first vertebra and the anterior third of the complex centrum as far 
backwards as the insertion of the crescentic process of the tripus into the dorsal wall 
of the air-bladder, at which point its walls blend with the dorsal edge of the trans- 
verse membrane. The anterior wall of the sac is perforated by the inter- ossicular 
ligament as tlie latter process passes inwards from the anterior extremity of the 
tripus to its connection with the scaphiurn. In the Cyprinoid Fishes the cavity of 
the sac is in open communication anteriorly with the cranial cavity through the 
external atrial aperture and the foramen occipitale laterale (hypoglossal foramen). 
In Macroues, and probably in all Siluroid Fishes, on the contrary, the small size of 
the hypoglossal foramen, which is but sufficiently large to transmit the hypoglossal or 
first spinal nerve, and the complete closure of the external atrial aperture by the 
spatulate process of the scaphiurn, effectually prevent any communication between 
the cranial cavity and the saccus. 
The cavity of the sac is occupied by a semi-gelatinous fluid, often partly oleag^inous 
from the presence of an abundance of oil -globules, and contained within the meshes 
of a delicate fibrous network. The jelly-like material is nevertheless sufficiently 
liquid not appreciably to hanqjer the grosser movements of the various ossicles that 
are imbedded in it. 
The roots of the first four pairs of spinal nerves traverse the cavity of the saccus 
