740 
MR. A. SANDERS ON THE ANATOMY OF THE 
This arrangement corresponds to that described by Gegenbaur'" in Hexanchus, 
although there is a slight difference in detail in the distal distribution. In Hexanchus 
there appear to be a greater number of branchial arches, so there are a greater 
number of these rami branchiales. Moreover, the third branch, the ramus pharyngeus, 
seems to be absent in Pdiina, as I did not find it. 
In Scyllium (fig. 1) this nerve arises by six roots, which are placed close together 
with no interval between ; the most anterior of these roots is the largest and folds 
over the others, forming nearly half the nerve. After passing through the foramen the 
nerve develops an elongated ganglion, from which four branches are given off, correspond - 
ing to those in Ithina. In the distal arrangement, however, the rami branchiales differ 
in not bifurcating ; but each branch goes direct into the base of the posterior gill of 
its respective branchial arch, so that they are represented solely by the rami 
pharyngei; in other respects the distribution in both species is alike. Acanthias 
differs from the other species in this that the vagus, instead of arising by several small 
roots, arises by one large one placed some distance behind the origin of the glosso¬ 
pharyngeal. 
Eleventh pair.—The hypoglossal does not appear to be present in any of the 
specimens examined by me. 
The spinal nerves have the usual dorsal and ventral roots ; in the Rays only do they 
present any peculiarity, the anterior nerves, to the number of seventeen or more, 
come off close together in a small space beyond the skull. This arrangement, of 
course, has reference to the largely developed lateral fins, which they supply; they 
are much larger in size and closer together than in the other species examined. 
The Central Cavities. 
The central cavities, canals, and ventricles are lined throughout by a layer of endo¬ 
thelium, which is also termed ependyma by some writers. 
The shape of the central canal in the spinal cord (figs. 6. 7, and 8) varies somewhat 
in Scyllium, but usually it is oval in section with the long axis placed transversely ; 
the transition to the fourth ventricle is indicated by a deficiency of the endothelium 
on the dorsal edge of the canal. The lumen gradually enlarges, in some cases 
becoming triangular or in others lozenge-shaped, the upper angle gradually extending 
to the dorsal surface, so that the canal at last becomes open at the top, forming the 
calamus scriptorius. There is often found a small quantity of granular material in the 
canal, which presents the appearance of being a coagulation ; it corresponds to the 
rod occasionally found in the central canal in the Teleostei, and shows perhaps that 
the cerebro-spinal fluid coagulates more firmly in the latter than in the former. 
The fourth ventricle (figs. 9, 10, 11, 12, and 13) is a broad shallow trough, extend¬ 
ing from the calamus scriptorius to the anterior end of the crura cerebelli ad medullam, 
* “ Ueber die Kopfnerven von Hexanchus.” ‘ .Tenaisclie Zeitschrift,’ vol. 6, 1871, p. 524. 
