IN GENERAL. 7 



hitherto indicated no renicarkable difference between them ; only 

 the one seems to contain less fat, a softer albumen \ and more water. 

 Tliis is of a grey colom- and receives a greater quantity of blood- 

 vessels than the other, which is white and is named medullary sub- 

 stance (comp. Vol. I. p. 11). This denomination and that of co7'tt- 

 cal substance are derived from the relative position of the two 

 substances in the brain, where usually the white substance is sur- 

 rounded by the grey. In the whole of the spinal marrow on the 

 contrary the cortical substance is situated internally. 



In vertebrate animals two nervous systems may generally be 

 distinguished, one for the vegetative, another for the animal life, 

 although many invertebrate animals also, as we have formerly 

 noticed, already indicate a commencement of this division '^. On 

 the other hand, in some vertebrate animals the nervus sympatliicus 

 is little developed, and is in part replaced by tlie vagus. The 

 nerves of vegetative life unite in irregular flat or round bodies 

 which are named nerve-ganglia. Here the branches of nerves form 

 many nets or plexuses, which principally surround the blood- 

 vessels. The whole is destitute of symmetry. 



In proportion to the larger development of the nervous system, 

 the organs of the senses in vertebrate animals are also more perfect. 

 Four of these, the organs of sight, hearing, smell and taste, are 

 situated in the anterior part of the head, included in bony cavities 

 and protected. The olfactory organ, formed by a folded mucous 

 membrane upon which the brandies of the first pair of nerves are 

 spread out, is in most fishes entirely segregated and placed on the 

 upper surface of the bony head, but in those vertebrates that breathe 

 by lungs is connected behind with the cavity of the mouth, and 

 so with the respiratory organs. The sense of feeling (and more 

 particulary that of tact) is in some animals especially developed 

 in the fingers, in others in the lips, in others again probably in the 

 tongue. Eyes are always present, as it would seem, in vertebrate 

 animals, although they are often very small and sometimes concealed 

 under the skin, and in certain fishes are two little balls surrounded 

 by pigment without refracting media, and so can have no other 

 office than that of distinguishing light. 



^ John Ckemische Tabellen des Thierreiclis, s. 

 ^ See Vol. I. p. 766. 



