SECT. lOQ.] 



NERVOUS SYSTEM. 



2T I 



This manner of viewing the Nervous System is, however, only 

 partially correct, because, 1st, in the so-called central organs very 

 many subordinate parts, as in the nerves, occur; and, 2nd, the 

 peripheral nervous system possesses, in the so-called ganglia or 

 nervous knots, likewise, physiological and anatomical central 

 organs. The old classification, also, of the nervous system into 

 animal and vegetative is no longer tenable when compared with 

 recent observations, and the latter, or the sympathetic or gang- 

 lionic nervous system, is to be regarded only as a part of the peri- 

 pheral nervous system, although peculiar in form. 



Elements of the Nervous System. 

 § 109. The nerve-tubes or nerve-fibres (fig. 90), also called pri- 



Fig. 90. 



mitive tubules, or primi- 

 tive fibres of the nerves, 

 are soft, fine, cylindrical 

 fibrils, of crooo5'" to 

 O'Oi'" in diameter, which 

 constitute the main mass 

 of the nerves and of the 

 white substance of the 

 central organs, but still 

 are not wanting in most 

 parts of the gray sub- 

 stance of the latter, and 

 in the ganglia. "When 

 examined fresh, and by 

 transmitted light, they 

 are crystalline and trans- 

 parent, with simple dark 

 contours ; by reflected 

 light, shining and opal- 

 escent, like fat, for the 

 most part white, and with 

 no outward indication 

 that they are composed 

 of different constituents ; 

 still by various methods 

 it is readily shown, that 

 they consist of three en- 

 tirely different structures, viz 

 and a soft but clastic fibre situate in the centre. 



p 2 



Nerve-fibres, magnified 3")0 times. 1. Of the dog and 

 rabbit, in the natural condition ; a. fine ; b. middling thick ; 

 c. thick fibre, from the peripheral nerves. 2. Of the frog, 

 with the addition ot serum ; a. drop forced out by pressure; 



b. axis cylinder in the same, continuing into the tube. 3. 

 Of the fresh spinal cord of man, with the addition of serum ; 

 a. investment ; 6. medullary sheath, with double contours; 



c. axis cylinder. 4. Double contoured fibre of the human 

 fourth ventricle, the axis cylinder (a) projecting and visible 

 in the fibre. 5. Two isolated axis cylinders from the cord, 

 the one undulated, the other unequaily thick, with adherent 

 medulla. 



a delicate envelope, a viscid fluid, 



