Page:Proceedings of the Royal Society of London Vol 4.djvu/485

true insects, and on the other with the Arachnida. The brain and the visceral nerves, the coverings and structure of the cord and ganglia, and the distribution of the systemic nerves are examined in each genus, but more particularly in the Scorpion, in which the nerves of the limbs are traced to the last joints of the tarsi, and those of the tail to the extremity of the sting. Especial attention is bestowed on the structure of the cord and its ganglia, and their de- velopement during the growth of the animal. In the lowest forms of the lulidse, in which the ganglia are very close together, and hardly distinguishable from the non-ganglionic portions of the cord, the author has satisfactorily traced four series of fibres, a superior, and an inferior one, and also a transverse and a lateral series. The superior series, which he formerly described in insects as the motor tract, he has assured himself is distinct from the inferior, which he regarded as the sensitive tract ; this evidently appears on examining the upper and under sides of a ganglionic enlargement of the cord. On the upper surface the direction of the fibres is perfectly longitu- dinal ; while the fibres on the under surface are enlarged, and cur- vilinear in their direction. But he remarks that it is almost impos- sible to determine by experiment whether these structures are sepa- rately motor and sensitive, as formerly supposed, or whether they both administer to these functions by an interchange of fibres. These two series appear also to be separated in each ganglionic enlargement of the cord by the third series, constituting the transverse or com- missural fibres, which pass transversely through the ganglia, and of which the existence was first indicated by the author in his paper on the Sphinx ligustri, published in the Philosophical Transactions for 1834. The author states that, in addition to these, there is in each half of the cord another and more important series of fibres, which constitute a large portion of the cord, but of which the existence has hitherto entirely escaped observation. This series forms the lateral portion of each half of the cord, and differs from the superior and inferior series in the circumstance, that while those latter series are traceable along the whole length of the cord to the suboesophageal and cerebral ganglia, the former series extends only from the posterior margin of one ganglion to the anterior margin of the first or second beyond it ; thus bounding the posterior side of one nerve and the an- terior of another, and forming part of the cord only in the interval be- tween the two nerves. From this circumstance, the author designates the fibres of this series,^6re5 of reiriforcement of the cord. Every nerve proceeding from a ganglionic enlargement is composed of these four sets of fibres, namely, an upper and an under one, communicating with the cephalic ganglia ; a transverse or commissural, which com- municates only with corresponding nerves on the opposite side of the body ; and a lateral set, which communicates only with nerves from another ganglionic enlargement on the same side of the body, and which forms part of the cord in the interspace between the gan- glia. The author had long suspected that this latter set of fibres existed ; but he had never, until lately, ascertained their presence by actual observation. Their action seems fully to account for the re-