IO$2 



VERTEBRATED ANIMALS 



vesicles. The cells, which do not seem to possess a definite cell- wall, 

 are, for the most part, composed of a finely granular substance, which 

 extends into their prolongations ; and in the midst of this is usually 

 to be seen a large well-defined nucleus. They also generally contain 

 pigment-granules, which give them a reddish or yellowish-brown 

 colour, and thus impart to collections of ganglionic cells in the 

 warm-blooded Vertebrata that peculiar hue which causes them to be 

 known as the cineritious or grey matter, but which is commonly 

 absent among the lower animals. Each of the tubular nerve-fibres, 

 on the other hand, of which the trunks are made up, consists, in its 

 fully developed form, of a delicate membranous sheath, within which 

 is a hollow cylinder of a material known as the ' white substance of 

 Schwann,' whose outer and inner boundaries are marked out by two 

 distinct lines, giving to each margin of the nerve-tube what is de- 

 scribed as a * double contour.' The contents of the membranous 

 envelope are very soft, yielding to slight pressure ; and they are so 

 quickly altered by the contact of water or of any liquids which are 

 foreign to their nature that their characters can only be properly 

 judged of when they are quite fresh. The 

 centre or axis of the tube is then found to 

 be occupied by a transparent substance 

 which is known as the ' axis cylinder ; ' 

 and there is reason to believe that this last, 

 which is a protoplasmic substance, is the 

 essential component of the nerve-fibre, 

 while the function of the hollow cylinder 

 that surrounds it, which is composed of a 

 combination of fat and albuminous matter, 

 is simply protective. The diameter of the 

 nerve-tubes differs in different nerves, being 

 sometimes as great as y^^th of an inch, 

 and as small in other instances as T^rou^h- 

 In many of the lower invertebrata, such as 

 Medusae and Comatulce, we seem fully 

 justified by physiological evidence in re- 

 garding as nerves certain protoplasmic 

 fibres which do not possess the characteristic structure of ' nerve- 

 tubes,' and fibres destitute of the ' double contour ' are found also 

 in certain parts of the body of even the highest vertebrates. These 

 fibres, which are known as ' gelatinous,' are considerably smaller 

 than the preceding, and do not exhibit any differentiation of parts 

 (fig. 789). They are flattened, soft, and homogeneous in their ap- 

 pearance, and contain numerous nuclear particles which are brought 

 into view by acetic acid. They can sometimes be seen to be 

 continuous with the axis-cylinders of the ordinary fibres, and also 

 with the radiating prolongations of the ganglion-cells ; so that their 

 nervous character, which has been questioned bv some anatomists, 

 seems established beyond doubt. 



The ultimate distribution of the nerve -fibres is a subject on 

 which there has been great divergence of opinion, and one which can 

 only be successfully investigated by observers of great experience. 



PIG. 789. Gelatinous nerve 

 fibres, from olfactory nerve. 



