340 CRUSTACEA. 



position above the oesophagus, and joins the abdominal centre by 

 two long cords of connection (j%. 161), gives off nerves to the 

 eyes and muscles connected with them, as well as to the antennae 

 and neighbouring parts. 



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 import- 

 ant branch, apparently the representative of the nervus vagus of 

 insects. This, after ramifying largely upon the coats of the sto- 

 mach, joins that of the opposite side ; and, assuming a ganglionic 

 structure, is ultimately lost upon the intestine. 



The nerves of the extremities, derived from the central abdo- 

 minal ganglion, are represented in the preceding figure (fig. 161), 

 which requires no explanation.* 



(375.) We have already ($ 313), when describing the nervous 

 system of insects, hinted at the probable existence in the HOMO- 

 GANGLIATA 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 corre- 

 spond with the sensitive and motor columns which have been dis- 

 tinctly 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 fact ;^ 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 sub- 

 stance 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 

 this subject, is derived from an examination of the manner in 

 which the nerves given off from the central axis take their origin ; 



* Vide Swan ; Comparative Anat. of the Nervous System. London, 4to. 

 t Phil. Transact. 1834. 



