NICOLAUS STENO'S NEW MYOLOGY (1667): RATHER THAN MUSCLE, THE MOTOR FIBRE SHOULD BE CALLED ANIMAL'S ORGAN OF MOVEMENT*

Abstracttitle ABSTRACT /title Muscular movement is the result of fibre shortening. How did this basic insight arise? Based on several of his observations, Nicolaus Steno in 1664 and 1667 proposed that muscles shorten when fibres shorten, and that skeletal musdes consist of uniform motor fibres layered as pennate structures. The basis for a new myology was provided in a geometrical model of the movement of the muscles. But fibre shortening was incompatible with the dominant ancient theory of contraction by inflation that was favoured by Descartes and by Steno's contemporaries William Croone, Thomas Willis, John Mayow, and Giovanni Borelli due to their adherence to the Aristotelian axiom: "Anything which moves is moved by something else". The inflation theory blindfolded researchers well into the eighteenth century for skeletal and heart muscles. When the shortening of motor fibres was eventually visualised by microscopy, this inflation theory was no longer tenable. Steno's structural daim on skeletal muscles was also rejected by Borelli and by later commentators. Pennate musdes were only rarely displayed until 1981 when macro-anatomical studies showed the morphology of most skeletal musdes to be similar to that described by Steno. Steno's proposals on muscles have since become a commonplace in computer models applied in the study of human and animal motion.

The Small Leg-Nerve of Spiders and a Probable Mechanoreceptor

Besides the main leg-nerve there is, in spiders, a ‘small leg-nerve’ which is mainly sensory. About 8 of the fibres arise from neurones which form a ganglion at the femurpatella joint and appear anatomically to be joint mechanoreceptors, but if so must supplement the lyriform organs. Cutting the small leg-nerve has no effect on the walking pattern nor on the detection of prey, which almost certainly depends on mechanoreception. The small leg-nerve contains one motor fibre which innervates the claw elevator. This muscle has a double innervation, a second fibre running in the main leg-nerve. There are interesting parallels between the small leg-nerve in spiders and Limulus.

The Reflex Mechanism of the Insect Leg

1. Variations in the frequency of the motor discharges to the leg muscles of Periplaneta americana are followed in the intact animal under different types of sensory stimulation by electrical recording from the muscles. 2. Two main reflexes are described: the depressor reflex, evoked by stimulation of the campaniform sensilla on the legs, and a levator response to touch on the upper side of the leg. 3. There is a direct antagonism in the excitation of the depressor and levator sets of muscles: also between similar muscles in the two legs of a segment. 4. Reflex effects are not transmitted up or down the animal to other segments. 5. The conditions for the reflex excitation of the two types of motor fibre are discussed. 6. A suggestion is put forward for the explanation of rhythmic movements.