This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1029). From a review of the above facts, Milne-Edwards and Audouin arrived at the following conclusions: - 1st. That the nervous system of Crustacea consists uniformly of medullary nuclei (ganglions), the normal number of which is the same as that of the segments or rings of the body. 2nd. That all the modifications encountered, whether at different periods of the development or in different species of the series, depend especially on the more or less complete approximation of these nuclei, and on an arrest of development in some of their number. 3rd. That approximation takes place from the sides towards the mesian line, as well as in a longitudinal direction.
(1030). In the Crab, the distribution of the nerves is briefly as follows: - The encephalic mass, or brain, which still occupies its position above the oesophagus, and joins the abdominal centre by two long cords of connexion (fig. 206), gives off nerves to the eyes, and muscles connected with them, as well as to the antennas and neighbouring parts.
(1031). Near the centre of each division of the nervous collar that surrounds the oesophagus is a ganglionic enlargement, from which arises a nerve that runs to the mandibles, and also a very important branch, apparently the representative of the nervus vagus of insects. This, after ramifying largely upon the coats of the stomach, joins that of the opposite side, and, assuming a ganglionic structure, is ultimately lost upon the intestine.
* "Untersuchungen uber die Bildung des Flusskrebses," Ann. des Sci. Nat. t. xx.
1 For a minute account of the arrangement of the nervous system in these animals, the reader is referred to the Cyclopeedia of Anatomy and Physiology, art. "Crustacea," by Dr. Milne-Edwards.
(1032). The nerves of the extremities, derived from the central abdominal ganglion, are represented in the annexed figure (fig. 206), which requires no explanation*.
Fig. 206. Nervous system of the Crab: from the dorsal aspect.
(1033). We have already (§ 858), when describing the nervous system of insects, hinted at the probable existence in the Homogan-gliata of distinct tracts of nervous matter in the composition of the central chain of ganglia, and in the filaments whereby they are connected with each other: reasoning therefore from analogy, it seems fair to presume that, if this be the case, such tracts correspond with the sensitive and motor columns which have been distinctly proved to exist in the spinal axis of vertebrate animals. It is to Mr. Newport that we are indebted for the first indication of this interesting fact1; and the accuracy of his observations is readily demonstrable by a careful examination of the ganglionic chain of the Lobster and other large Crustacean species. Each ganglionic enlargement is, upon close inspection, clearly seen to consist of two portions: first, of a mass of cineritious nervous substance forming the inferior aspect of the ganglion, and of a cord of medullary or fibrous matter which passes over the dorsal or superior aspect, and appears to be distinct from the grey substance over which it passes: supposing therefore the longitudinal chain to consist of anterior and posterior fasciculi, as in the medulla spinalis, we have the anterior columns communicating with grey substance, while the posterior are unconnected therewith, but are continued over the ganglion instead of becoming amalgamated with its substance.
Another fact which favours Mr. Newport's view of the subject is derived from an examination of the manner in which the nerves given off from the central axis take their origin; for some of them undoubtedly proceed from the cineritious portion of the ganglionic swelling, while others, derived from the upper column, not only have no connexion with the grey matter, but arise at some distance from the ganglionic mass: judging therefore by the laws at present established in physiology, there seems reason to suppose that the anterior, or, rather, inferior fasciculi are connected with sensation, while the superior constitute the motor tract.
* Vide Swan, Illustrations of the Comparative Anatomy of the Nervous System. London, 4to. 1 Phil. Trans. 1834.
(1034). The reader who is conversant with human physiology will at once perceive that this arrangement is precisely the reverse of that met with in Man and other Vertebrata; and this consideration, apparently of little importance, has given rise to a variety of curious speculations - some anatomists having even gone so far as to assert that all the organs of articulated animals are in reality placed in a similar inverted position.
(1035). A more interesting inquiry connected with this part of our subject is, concerning the extent to which the Articulata are susceptible of pain. Is it really true in philosophy, as it has become a standing axiom in poetry, that "The poor beetle, that we tread upon, In corporal sufferance feels a pang as great As when a giant dies"?
(1036). This is a question upon which modern discoveries in science entitle us to offer an opinion; and the result of the investigation would seem to afford more enlarged views relative to the beneficence displayed in the construction of animals than the assertion of the poet would lead us to anticipate. Pain, "Nature's kind harbinger of mischief," is only inflicted for wise and important purposes - either to give warning of the existence of disease, or as a powerful stimulus prompting to escape from danger. Acute perceptions of pain could scarcely, therefore, be supposed to exist in animals deprived of all power of remedying the one or of avoiding the other. In Man, the power of feeling pain indubitably is placed exclusively in the brain; and if communication be cut off between this organ and any part of the body, pain is no longer felt, whatever mutilations may be inflicted.