| OCR Text |
Show 884 MR. WALTER E. COLLINGE ON THE SENSORY [Dec. 17, exact relations to the facial. A commissural connection between the trigeminal and vagus is present in Ceratodus (cf. Sanders, 14), in Lepidosiren (?) \ and between the facial and vagus in Protopterus (cf. Pinkus, 13). The commissure does not join the ganglion but the ramus vagi lateralis at its commencement slightly beyond the ganglion, as in Ceratodus, after which it passes beneath the muscles lying close to, and parallel with, the vertebral column (a feature common to Lepidosiren also) at the side of the centra and dorsal to the spinal nerves. It passes backwards to almost the end of the whip-like prolongation of the tail, becoming smaller posteriorly. Fine branches pass off ventrally at intervals, the distance varying from five to twelve millimetres-each branch making a ventral curve and passing dorsally again to the lateral canal, there breaking up into a series of fine fibres. The remaining branches of the vagus are not concerned with the sensory canal-system and call for no special mention. III. THE AMPULLARY CANALS. 1. General Description, One of the most interesting features in Chimcera is the presence of groups of Ampullary Canals. They were first described in this fish by Leydig (12). There are in Chimcera five main groups on either side of the head, which from their position and for reference may be termed respectively:- 1. The Occipital group), situated anterior to that portion of the main canal which leads to the occipital commissure. 2. The Median group, a series lying on either side of the mid-dorsal line of the head. 3. The posterior and anterior Supra-orbital group, situated below the supra-orbital branch of the main sensory canal of the head. 4. The posterior and anterior Sub-orbital group, situated below and above, respectively, tbe sub-orbital branch of tbe main canal of the head. 5. The posterior and anterior Maxillo-mandibular group, situated above the maxillo-mandibular branch of the main canal of the head. The posterior and anterior groups of the supra- and sub-orbital groups are often continuous and in some cases one or both were absent, on either one or both sides of the head: in such cases there were slight differences in the branching of the nerve (cf. 4) The position of these canals is by no means so constant as in the Elasmobranchs, and isolated lines or clusters are frequently found in other positions than those noted above. They are always distinct from the sensory canal-system. 1 From the examination I have made of Lepidosiren I could not positively say whether this commissure arises from tbe trigeminal or facial, but, from its position outside the cranial cavity, I think the former. |
