THE NERVOUS SYSTEM—GENERAL CONSIDERATIONS. 911 
thelial covering of the body (end-organ) in which a nerve ter- 
minates, which latter may be traced to a cell or cells removed 
from the surface (center), and from which in most cases other 
nerves proceed, 
The nervous system, we may remind the student, consists in 
vertebrates of centers in which nerve-cells abound, united by 
nerve-fibers and by the most delicate form of connective tissue 
known, in connection with which there are incased strands of 
protoplasm or nerves as outgrowths. The main centers are, of 
course, aggregated in the brain and spinal cord. 
It is possible to conceive of the work of a nervous system 
carried on by asingle cell and an afferent and efferent nerve; 
but inasmuch as such an arrangement would imply that the 
central cell should act the part of both receiving and origi- 
nating impulses (except it were a mere conductor, in which case 
there would be no advantage whatever in the existence of a cell 
at all), according to the principle of the physiological division 
of labor, we might expect that there would be at least two cen- 
tral cells—one to receive and the other to transmit impulses— 
or at least that there should be some specialization among the 
central cells; and we shall have good reason later to believe 
that this has reached a surprising degree in the highest ani- 
* mals, 
Moreover, it would be a great advantage if the termination 
of the ingoing (afferent) nerve should not lie exposed on the 
surface, but be protected by some form of cell that had also the 
power to transmit to it the impressions received from without, 
in a form suitable to the nature of the nerve and the needs of 
the organism. 
So that a complete mechanism in its simplest form would 
furnish: 1, A peripheral cell or nerve end-organ, 2. An affer- 
ent or sensory nerve. 3. Two or more central cells. 4. An 
efferent nerve, usually connected with—5. A muscle or other 
form of cell, the action of which may be modified by the out- 
going nerve, or, as we should prefer to say, by the central nerv- 
ous cells through the efferent nerve. The advantages of the 
principal cells being within and protected are obvious. 
When, then, an impression made on the peripheral cell is 
carried inward, there modified, and results in an outgoing nerv- 
ous impulse answering to the afferent one, giving rise to a mus- 
cular contraction or other effect not confined to the recipient 
cells, the process is termed reflex action. 
The great size, the multiplicity of forms, the distinct out- 
