Picture - Cosmic Polymath

BEGINNINGS OF NERVOUS SYSTEM 135
So long as the nervous system is intact these messages are possible. If, however, any part of the nervous
system be broken down by injury or disease, the messages become unintelligible or altogether cease. The same thing
happens when there is a break-down in the telegraphic system.
It will be convenient to discuss this important subject somewhat more fully, especially in relation to the nerve
elements themselves.
That the ganglia of the spinal cord with their complement of sensory and motor nerves can act independently
of the brain can scarcely be doubted. Such action, however, implies no irritability on the part of the ganglia and
nerves of the cord, and no extraneous stimulation. It simply means that the cord with its ganglia and sensory
and motor nerves may be regarded as an independent nerve centre to the more or less complete exclusion of the
brain. The gangUa or nerve centres of the cord can receive sensory impressions on their own account. They can
also convert sensory impulses into motor impulses. This they can do at first hand, and without extraneous interven-
tion. The history of the ganglia or nerve centres, spinal cord, and brain, makes this abundantly clear. The ganglia
or nerves centres can act without a spinal cord, and the spinal cord can act without a brain, when the latter is
diseased or removed. The object of the nerve centres in every instance is, firstly, to co-ordinate and bring into
line the several parts of the body by means of more or less perfect voluntary movements ; and secondly, to
connect the animal with the outside world. This the nerve centres do by the aid of ganglia and sensory and motor
nerves ; the former going to, the latter coming from them. No hypersensitiveness or excitabiUty of the nerve
centres, and no extraneous stimulation of them is required for their normal action.
In the jelly-fish, which is void of a brain, even the ganglia and sensory and motor nerves are rudimentary
yet the animal is capable of feeUng and acting voluntarily. In the starfish, as has been shown (and the same is true
of the aplysia), there is still no trace of a brain, but the ganglia or nerve centres and the sensory and motor nerves
are more fuUy developed. The starfish and aplysia can certainly feel and move voluntarily and to given ends as
apart from irritability and artificial stimulation. In the centipede the nervous system is arranged symmetrically
in two longitudinal lines ; each segment of the animal being provided with two gangha and two sets of sensory
and motor nerves : the two cephahc gangUa running together to form a rudimentary brain. The power of the
centipede to feel and move voluntarily goes without saying. In the double linear chain of gangha forming the
nervous system of the centipede can be traced the first beginnings of a spinal cord and brain. As a matter of
fact, the brain, even in man, is to be regarded as an expansion and elaboration of a bilaterally symmetrical spinal
cord. Virtually the same nerve elements enter into the composition of both. Both the brain and spinal cord
are provided with ganglia or nerve centres and with nerve commissures and sensory and motor nerves in great
profusion. They are also provided with an ample supply of conducting nerve fibres. The nerve commissures
connect the gangUa longitudinally and transversely to secure united, harmonious action. The gangha or nerve
centres, the spinal cord, and the brain are furnished with an abundant supply of rich blood, and the cord and brain
are provided with fibrous and osseous coverings to protect them from injury.
In the fishes and reptiles, the gangha which largely form the brain consist of a double chain or series, and
these are arranged on the same plane and on a line with the spinal cord itself. It is only in the birds and mammals,
especially the latter, that the cerebral lobes or hemispheres (cerebrum) of the brain sprout upwards umbrella-fashion,
and cover in more or less completely from above the double chain of gangha which, as explained, are to be regarded
as continuations of the ganglia of the spinal cord itself.
These several points are illustrated at Plates Ivii. and Iviii.
A subject of pecuhar interest in this connection is the nature of the gangha or nerve centres themselves,
whether regarded as isolated, independent entities as they occur in animals, with no spinal cord and no brain ; or
as they occur in animals, with a spinal cord and a rudimentary brain ; or as they are found in animals, with an
elaborate spinal cord and with a wonderfully complex and highly differentiated brain. A microscopic examination
some being round,
of the gangha or nerve centres, and the cells composing them, shows them to be variously shaped ;
some oval, some triangular, and some stellate. The star-shaped appearance is, for the most part, due to the entrance
and exit of the sensory and motor nerves or conducting nerve substance of some kind ; the points at which the
sensory nerves enter and the motor nerves leave the nerve centres being known as poles. The gangha, whether
they belong to the cerebro-spinal or sympathetic system of nerves, consist of grey matter, the darker nerve
substance. It occupies a central position in the spinal cord, and a peripheral one in the brain. This grey matter dis-
charges very important functions, being accredited with the power of thinking in the brain, and of converting sensory
into motor impulses in the spinal cord. The conducting nerves of the spinal cord and brain are composed of white
nerve substance. The nerve gangha are, in many cases, highly elaborated and exceedingly complex (Fig 2, A, B of
Plate Ivii. and also 1, 2, 3, 4 of Plate Iviii.). They consist, hke ordinary cells, of a cell wall or envelope, a nucleus,
and a nucleolus—the contents of the nerve cell being molecular, granular, and protoplasmic in character. It is in