Chap. V.] GENERAL HISTOLOGY. 75 



most of the fibres of the sympathetic system. Such fibres still 

 retain the primitive sheath. But eventually even this may be 

 lost, and the axis fibre may split into fine fibrillae which may 

 terminate in specially-modified cells. In the optic and auditory 

 nerves there is a medullary sheath, but no primitive sheath, and 

 no primary nodes. 



(b.) Nerve cells. Traced inwards to the spinal cord a nerve 

 is found to result from the union of two roots, a ventral or motor 

 root, and a dorsal or sensory root. On the latter is a swelling or 

 ganglion. Here there are a number of unipolar nerve cells (26, x.) 

 Each is invested with an investing sheath continuous with the 

 primitive sheath. An axis fibre may, in some cases, be seen to be in 

 connection with the cell, and to bifurcate at some little distance 

 from it. In the ganglia of the sympathetic system of the frog, 

 some bipolar cells of the type figured in 26, ix. may be found. 

 The straight axis cylinder process passes into a non-medullated 

 fibre : the spiral fibre, after leaving the axis cylinder, acquires a 

 medullary sheath and becomes a medullated fibre. 



On the ventral root of the spinal nerve there is no ganglionic 

 enlargement. The fibres pass inwards into the spinal cord, and 

 there come into relation with multipolar ganglion cells such as 

 that figured in 26, xi. 



9. The Spinal Cord and Brain. In Fig. 26, xii. is figured a 

 section of the spinal cord (from the cervical region of a kitten), 

 showing the origin of the dorsal and ventral nerve roots. In the 

 middle of the cord is a canal (canalis centralis) lined with ciliated 

 epithelium. Above and below this the cord is nearly cleft in 

 twain by a dorsal and a ventral fissure, into the latter of which 

 pass bands of connective tissue derived from the investing mem- 

 brane or pia mater. The substance of the cord is composed of 

 white matter externally, and grey matter internally. The white 

 matter consists of medullated nerve fibres without any primitive 

 sheath and interrupted by no nodes. They run longitudinally, 

 but form also commissures, passing from one half of the cord 

 to the other. The grey matter, besides having somewhat fewer 

 medullated fibres and more blood-vessels, has also, as a char- 



