270 ANATOMY. 



derably more multifarious than in any other invertebrate animal ; yet 

 it does not unfold itself to that universal intercourse found in the 

 superior animals. But they are nevertheless sensible to every possible 

 external impression, and indeed for many more sensibly so than the 

 class of fish immediately above them, which, however, and this supports 

 the above assertion, are provided with distinct organs of hearing and 

 of smell, which are wanting in insects, although they require them 

 much more in the so considerably more tenuous element they inhabit, 

 than the fish, which pass their lives as it were concealed. 



It is thence evident what we understand by organs of sensation, 

 namely, all forms which may be considered either as direct conductors 

 of immediate feelings, or as the recipients of higher and more distant 

 perceptions. To the first we may class the nerves, to the last the organs 

 of the senses, and in insects especially, the eye. 



The nerves, which are the foundation of all the organs of sensation, 

 consist of fine fibres, which appear to be composed of the consecutive 

 disposition of solid globules. These atoms, from which all nerves 

 appear to be originally formed, preponderate so much in insects, that 

 we never detect in the ganglia and in the nervous cords but rarely 

 a fibrous formation, which would admit of the conclusion of its being 

 formed of a concourse of individual threads. The nervous mass is 

 contained within a very delicate structureless and perfectly transparent 

 membrane, the nervous sheath (iieurilemu) , which appears to be the 

 mould of the entire nervous system, at least in insects. In it the 

 nervous mass is enclosed, which is a soft pulpy substance which flows 

 out when the sheath is opened. Upon a first superficial examination, 

 the chief nervous cords of insects, at least both the large ventral cords, 

 appear to be formed of several contiguous fibres, parallel stripes being 

 observable in them ; but these disappear upon a closer inspection, and 

 each nervous cord is found to be nothing else than a tube formed 

 of the nervous sheath filled with the nervous mass. The apparent 

 striature proceeds from the globules not being irregularly placed, but 

 disposed in longitudinal rows. Thus individual nervous cords appear, 

 and they even become so when, as in the superior animals, the 

 mass thickens, and thereby presses the globules together, and the 

 neurilema falls down between the striae. 



The nervous mass itself consists of two different substances, namely, 

 the firmer, white central mass, and the softer, darker-coloured cortical 

 substance, and which is sometimes of a beautiful carmine, according to 



