BLUM The ganglions are small masses occupying certain situations in the body. They are ex tremely numerous in the hurnan body, and very variable in shape and size. One great sub division of them, in man and the mammalia, is connected with the posterior roots of the spinal, and with certain encephalic nerves. Another class belongs to the sympathetic sys tem. In the Invertebrata the nervous system is made up of a series of them variously dis posed, with their afferent, efferent, and con necting nerves.
The spinal cord and the brain are peculiar to the great class of vertebrated animals. They may be regarded as compound ganglions, being physiologically resolvable into a series of smaller centres, which are, to a certain extent, inde pendent of each other. Viewed anatomically, they are not so obviously divisible: in the spinal cord, in which the independent influence of separate segments may be most easily demonstrated, no anatomical subdivision is obvious, for the segments are fused together into a cylindroid body, which has a certain relation to the length and muscular activity of the animal. Indications, however, of this composite form of the spinal cord are afforded, in the marked difference of dimensions which certain parts of it present when compared with others, there being always a manifest corres pondence between the size of any segrnent of the cord and the rnotor or sensitive endowment of that segment of the body which receives its nerves from it. And the case of the common gurnard (Trigla Lyra) may be here quoted as a remarkable instance of the developement of distinct gangliforrn bodies on a portion of the cord, in accordance with a particular exaltation of tactile sensibility.
The brain is much more evidently made up of a series of separate centres or smaller masses, exhibiting sufficiently distinct boundaries on their surfaces, but so intimately connected by what are called commissural or uniting fibres, as to manifest the same kind of fusion (although to a less degree) as that noticed in the spinal cord. These gangliform bodies are so readily disting,uishable from one another, that from the earliest periods of anatomical investigation each of them has been designated by a distinct name, which is generally derived from some prominent feature of the body itself, or frorn the name of some familiar object which it has been sup posed (often fancifully) to resemble. The aggregate of these bodies is known in popular lang,uPge by the name of Brain, word of Saxon origin, sonietimes used in the plural); this word, however, anatomically speaking, is applicable only to the great hemispheric lobes which form the largest portion of the whole mass; and the term Encephalon may be more correctly used to denote the whole of the intra cranial contents.
It is proposed in the present article to con sider the geneml and descriptive anatomy of these nervous centres severally, beginning with an exarnination of their coverings.