MYOLOGY.
In the lowest vertebrate, _Branchiostoma_, the whole of the
_muscular_ mass is arranged in a longitudinal band running along
each side of the body; it is vertically divided into a number of flakes
or segments (_myocommas_) by aponeurotic septa, which serve as the
surfaces of insertion to the muscular fibres. But this muscular band
has no connection with the notochord except in its foremost portion,
where some relation has been formed to the visceral skeleton. A very
thin muscular layer covers the abdomen.
Also in the _Cyclostomes_ the greatest portion of the muscular
system is without direct relation to the skeleton, and, again, it is
only on the skull and visceral skeleton where distinct muscles have
been differentiated for special functions.
To the development of the skeleton in the more highly organised fishes
corresponds a similar development of the muscles; and the maxillary
and branchial apparatus, the pectoral and ventral fins, the vertical
fins, and especially the caudal, possess a separate system of muscles.
But the most noteworthy is the muscle covering the sides of the trunk
and tail (already noticed in _Branchiostoma_), which Cuvier described
as the “great lateral muscle,” and which, in the higher fishes, is a
compound of many smaller segments, corresponding in number with the
vertebræ. Each lateral muscle is divided by a median longitudinal
groove into a dorsal and ventral half; the depression in its middle
is filled by an embryonal muscular substance which contains a large
quantity of fat and blood-vessels, and therefore differs from ordinary
muscle by its softer consistency, and by its colour which is reddish
or grayish. Superficially the lateral muscle appears crossed by a
number of white parallel tendinous zig-zag stripes, forming generally
three angles, of which the upper and lower point backwards, the middle
one forwards. These are the outer edges of the aponeurotic septa
between the myocommas. Each septum is attached to the middle and the
apophyses of a vertebra, and, in the abdominal region, to its rib;
frequently the septa receive additional support by the existence of
epipleural spines. The fibres of each myocomma run straight and nearly
horizontally from one septum to the next; they are grouped so as to
form semiconical masses, of which the upper and lower have their apices
turned backwards, whilst the middle cone, formed by the contiguous
parts of the preceding, has its apex directed forward; this fits into
the interspace between the antecedent upper and lower cones, the apices
of which reciprocally enter the depressions in the succeeding segment,
whereby all the segments are firmly locked together (_Owen_).
In connection with the muscles reference has to be made to the
_Electric organs_ with which certain fishes are provided, as it
is more than probable, not only from the examination of peculiar
muscular organs occurring in the Rays, _Mormyrus_, and _Gymnarchus_
(the function of which is still conjectural), but especially from the
researches into the development of the electric organ of _Torpedo_,
that the electric organs have been developed out of muscular substance.
The fishes possessing fully developed electric organs, with the power
of accumulating electric force and communicating it in the form of
shocks to other animals, are the electric Rays (_Torpedinidæ_), the
electric Sheath-fish of tropical Africa (_Malapterurus_), and the
electric Eel of tropical America (_Gymnotus_). The structure and
arrangement of the electric organ is very different in these fishes,
and will be subsequently described in the special account of the
several species.
The phenomena attending the exercise of this extraordinary faculty
also closely resemble muscular action. The time and strength of the
discharge are entirely under the control of the fish. The power is
exhausted after some time, and it needs repose and nourishment to
restore it. If the electric nerves are cut and divided from the brain
the cerebral action is interrupted, and no irritant to the body has any
effect to excite electric discharge; but if their ends be irritated
the discharge takes place, just as a muscle is excited to contraction
under similar circumstances. And, singularly enough, the application
of strychnine causes simultaneously a tetanic state of the muscles and
a rapid succession of involuntary electric discharges. The strength of
the discharges depends entirely on the size, health, and energy of the
fish: an observation entirely agreeing with that made on the efficacy
of snake-poison. Like this latter, the property of the electric force
serves two ends in the economy of the animals which are endowed with
it; it is essential and necessary to them for overpowering, stunning,
or killing the creatures on which they feed, whilst incidentally they
use it as the means of defending themselves from their enemies.