CEREBROSPINAL NERVOUS SYSTEM. 419 
ganghonic neuron, with its two axons and no typical dendrites, is a nervous unit 
very different from a neuron in the gray matter of the cerebrospinal axis. It is 
beheved by some, however (van Gehuchten and Cajal), that the peripheral process, 
in spite of its enclosure within a medullary 
sheath, and though presenting all the char- 
acters of a true axon, is in reality a dendrite. 
If this be the case, the morphological differ- 
ence between a dendrite and an axon dis- 
appears, and van Gehuchten’s functional 
distinction alone remains characteristic, viz. 
that the axon is cellulifugal and conducts 
impulses away from the cell, whilst the 
dendrites are cellulipetal and conduct im- 
pulses towards the cell. 
Neuroglia.—The neuroglia is the sup- 
porting tissue of the cerebrospinal axis. It 
may be considered to include two different 
forms of cells, viz. the hning ependymal cells 
and the neuroglia cells proper. We place 
these cells under the one heading, seeing that 
in all probability they both have a common 
developmental origin. 
The ependymal cells are the columnar 
epithelial cells which line the central canal 
of the spinal cord and the ventricles of the |, .,. RIG PO  s 2ts9yen ea 
: : Bic IG. 312.—THREE STAGES IN THE DEVELOPMENT 
brain. In the embryonic condition a process Gata Co on oman erivars CuINnaTTOt 
from the deep extremity of each cell traverses 
the entire thickness of the neural wall and reaches the surface. It is not known 
whether this process exists in the adult. 
The neuroglia cells proper are found in great numbers scattered throughout 
the gray and white substance of the brain 
and cord. They present many different 
forms, but the leading and distinguishing 
feature consists in the “great number of fine 
branching processes which as a rule pass 
out from all parts of the body. These 
neuroghial cells act as a sort of packing 
material placed between the nervous ele- 
ments, and their processes twine around the 
different parts of the neurons in such a 
way as both to isolate them and bind them 
together. 
The ependymal cells are undoubtedly 
derived from the original neuro-epithelial 
cells of the early neural tube, and in all prob- 
ability the neuroglia cells proper have a 
similar origin. They both, therefore, proceed 
from the ectoderm. 
Summary.—1. The cerebrospinal nervous 
Fig. 313. —Section THROUGH THE CENTRAL System is composed of two parts, viz. (@) a 
He SENeL COND) oy £ HUMAN medullary part, consisting Of they brain and. 
cL OL ee pee NEUE spinal cord, with the efferent nerve -fibres 
A, Ependymal cell. B, Neuroglial cell. which pass out from them; (b) the ganglionic 
part, with the afferent nerve-fibres. 
2. Each of these parts has a different origin and is composed of neurons which 
possess characteristic features. 
3. The ganglionic neurons are derived from the primitive cells of the neural 
crest, and have each one process which divides into two. Of these the central 
division enters the cerebrospinal axis and probably represents the axon, whilst the 
