Page:Illustrations of the comparative anatomy of the nervous system.djvu/25

 PLATE IIl.

DORSAL ASPECT OF THE LOBSTER.

(ASTACUS MARINUS.)

THE brain is seen giving off the optic nerves, and others to the contiguous parts and the feelers, and sending a long nerve on each side of the cesophagus, which gives filaments to the stomach and joins its fellow at the first ganglion: the nervous chord is then continued from this ganglion to the next, and so on to the tail. The chord exhibits more distinct and continued parallel lines on the dorsal surface than on the ventral. The large artery, supplying the extremities, &c., is seen passing between the two halves of the chord, just above the ganglion giving off the nerves to the last pair of legs. Nerves are seen issuing from the ganglia and the chord, to be distributed to the contiguous parts. In this plate the origins of the nerves of the legs are obscured by others passing over them; but their continuations to the tips of the claws are seen. The distinction of the origins of nerves from the dorsal or ventral surface is by no means clear, and cannot be compared with that in most of the vertebrated animals.

In many of the invertebrated animals the dissection may be generally made as for the preparation from which this plate was taken. The dorsal portion of the parietes may be removed, except a small part at the back of the head, which must be carefully cut away afterwards. For a hard shell, a pair of strong cutting forceps must be used; and, for others, common scissors. The brain is placed in the dorsal portion of the head, just above the cesophagus; and some care is necessary in tracing the nerves passing from it on each side of this canal to the first ganglion. The cesophagus is frequently very small, and easily divided; and may sometimes be best observed by introducing a bristle from the mouth. The nervous chord and ganglia will be found in the middle of the ventral parietes, after the superincumbent parts have been carefully separated.