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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- 



