176 
Proceedings of Royal Society of Edinburgh. [sess. 
Fig. 7. Vertical transverse section through the lower extremity of the superior 
nucleus (III. s.n .) or “small-celled nucleus” of Darkschewitsch, the upper 
part of the pale nucleus (III. p.i.n.), and of the anterior nucleus (III. a.n.). 
The anterior nucleus (III. a.n.), still futher reduced in size, lies internal to 
the anterior part of the posterior longitudinal fasciculus. Behind and partly 
internal to the anterior nucleus is seen the pale nucleus (p.i.n.), in a position 
corresponding to that described in the previous section. It is of somewhat 
larger size, but its component cells show otherwise similar appearances. 
The upper nucleus (II. s.n.) lies imbedded amongst the most posterior fibres 
of the posterior longitudinal fasciculus. It is somewhat ovoid in shape, the 
cells are distinctly smaller than those of the posterior nucleus, and there is a 
somewhat dense outer cellular network of medullated fibres. The posterior 
longitudinal fasciculus is considerably reduced in size compared with figs. 5 
and 6. 
Fig. 8. Transverse section through the superior nucleus (III. s.n.) p.l.f., 
the posterior longitudinal fasciculus greatly reduced in size; p.e., the 
posterior commissure passing forwards to terminate (partly) in the small-celled 
nucleus. 
Fig. 9. Longitudinal section through the III. nucleus. III. p.i.n., the 
postero -internal nucleus. Its club shape and its pallor are well represented — 
the large head above the narrower part below, p.l.f. posterior longitudinal 
fasciculus ; p.e.n., the postero-external (and probably partly anterior) nucleus ; 
III. vent., third ventricule. 
On the Nature of a Voluntary Muscular Movement. 
John Berry Hay craft, M.D., D.Sc. 
By 
(From the Physiological Laboratory of the University of Edinburgh. ) 
(Bead January 20, 1890.) 
(Abstract.) 
Histologists have long ago demonstrated that a muscle is not a 
simple histological unit, and that it consists of innumerable muscle 
fibres associated together in fasciculi, each muscle fibre being brought 
into individual relationship with the central nervous system by its 
own fibre. Physiologists have been too apt to overlook this fact, 
and have regarded a muscle as a simple indivisible physiological 
unit. When a muscle or its nerve are stimulated by an instantaneous 
electrical shock all the fibres contract together, for they are all 
stimulated in the same way, and at the same time. This is not the 
case, according to the investigations of the author, when the muscle 
contracts reflexly or voluntarily, and especially during any pro- 
longed contraction. The individual fibres are then never completely 
co-ordinated together, and vary constantly in their “pull.” They 
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