174 GENERAL ORNITHOLOGY. 
F 
b. NevuROLOoGY; THE NeRvouS SYSTEM; ORGANS OF SPECIAL SENSES. 
The Nervous System of any Vertebrate determines the form of such an animal; in fact, 
the beautiful skeleton we have examined is simply asketch in bone of the cerebro-spinal nervous 
system, conuformably with which the whole bony framework of the body is ereeted. A brain 
and spinal chord and their lateral prolongations or nerves aré the commanding superad- 
ditions, in a vertebrate, to any such nervous system as an invertebrate may or does possess. 
Besides the vertebrate or main nervous system, all brainy vertebrates retain a sympathetic 
system of nerves, supposed to represent a modified inheritance of the whole nervous system of 
Invertebrates. Thus the cerebro-spinal and sympathetic are the two distinct nervous systems 
of nearly all vertebrates, — of all vertebrates which have askull and brain. The former presides 
over the animal life of the creature, — its sensations, perceptions, and voluntary actions ; the 
latter more especially over its vegetative functions, as digestion, respiration, circulation, and 
reproduction, which are more or less involuntary. But the two are inseparably connected, 
anatomically and physiologically, so that no distinct line can be drawn between them. 
Nerve-tissue consists of an aggregation of nerve-cells and their investing substance, — the 
bodies of a myriad Newramebe agglutinated by their secretions. ‘They are of two species: 
Neurameba cinerea and N. candida. The former are usually multiradiate, inosculating cells 
of nerve-substance, which form the ‘‘ gray matter” of the brain and spinal chord and the 
ganglia (knots) of nerves; the latter are white, thready, and form the connections of the 
ganglionic masses and the whole substance of ordinary nerve-chords. The gray ameebas are 
the immediate communicants between the mind and the body of the creature; the white 
amcebas are the mediators between the body and outward things. The gray ameebas translate 
thought in terms of matter, and conversely; the white convey the translation. How this is 
done, no one knows, but the fact is manifest. In ordinary language, gray nerve centres receive 
from white tracts impressions made upon the periphery of the nervous system; and, with or 
without the knowledge and consent of the animal, convert these impressions into appropriately 
responsive actions. This is called the ‘“ retlex action” of the nervous system. Some think 
such reflectior. is the principal or only activity of the nerve-tissue, taking animals to be mere 
automata, the mechanism of which is only set in motion by external stimulation. Others think 
that animals, and even human beings, have in their consciousness an inner spring of action, 
vaguely called ‘ spiritual,” whose operations upon the matter of their bodies manifests what is 
called by some “‘ mind,” by others ‘‘ soul.” Iam satisfied of the correctness, in the main, of 
the latter view; but, however this may be, it is quite certain that white nerve tissue is a means 
of carrying something to and fro, which something is called a ‘nerve impulse,” for want of 
knowing what it is. White nerves have therefore an efferent function, when they carry im- 
pulses outward from gray centres, and an afferent function, when they bring impulses in to gray 
centres. The former is their motor function ; the latter is their sensory function. In nerves at 
large, impulses of both kinds travel in the same tracts without interference ; such mixed nerves 
are therefore called sensori-motor. Thus, each spinal nerve has a posterior sensory ganglion- 
ated root, and an anterior motor simple root, which soon blend in one chord, in which both 
functions coexist. Some nerves seem to be entirely motor, as those which move muscles of the 
face and tongue. The purest sensory nerves are those of “‘ special sense,” as the olfactory, 
optic, and auditory. Some nerves are so ‘‘ mixed” as to combine functions of special sense, 
common sensation, and motion, as that called glosso-pharyngeal, which moves, feels, and 
tastes. The motor eflluence of nerve tissue upon itself and other parts of the body is literally 
animation; the sensory influence is nominally materialization. The physical mechanism of 
these occult processes in a bird is as follows : — 
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