But Dr. Hall has fixed the attention of phy siologists on this class of facts, and has illus trated by experiments their independence of the Brain, and dependence on the Spinal Cord exclusively, and in this conclusion he is sup ported by many facts previously recorded by Le Gallois, Magendie, Flourens, and others.
It is further to be observed, that the contrac tions of voluntary muscles, which are supplied by thP nerves of the Symmetrical class of Sir C. Bell, while they are excited through the one set of filaments comprising those nerves, are made known to our consciousness by the others or sensitive filaments, and constitute the im portant class of Muscular Sensations. Of the movements of the strictly involuntary muscles, the heart, stomach, and bowels, and even the bladder, (supplied by irregular nerves,) we have, in the perfectly healthy state, no intima tion, although they frequently become percepti ble to us in disease, or when over-excited. But contractions of some of these involuntary mus cles also are pretty certainly excited by certain Sensations, as, e.g. a certain degree of antipe ristaltic movement in the stomach by the feel ing of nausea, and a certain movement of the pharynx and cesophagus by the sensations in the fauces, which prompt the act of deglutition ; and in: such cases, although not attended with consciousness, they are in all probability excited through the nerves of these muscular parts. Accordingly, the pharynx and cesophagus have been observed by Mr. Mayo, and the stomach by Breschet, Milne Edwards, and others, to be exceptions to the general rule of involuntary muscles being inexcitable by irritation of their nerves.
The old distinction of muscles into Volun tary, Involuntary, and Mixed, is very deficient in precision, so far as the last class is concerned. The true distinction is, of muscular contrac tions, into those excited in the natural state by Mental Stimuli, and through the intervention of Nerves (qui soli in corpore mentis sunt mi nistri)—and those excited by Physical Stimuli, acting on the muscles themselves, whereas the intervention of nerves is a theory, not an esta blished fact. The first class admits obviously, from what has been stated, of a division into movements excited by the Will, which depend on the Brain, and movements excited by invo luntary mental acts, especially by Sensations, which depend only on the Spinal Cord and medulla oblongata. The Will acts only on
muscles provided with sensitive nerves, by which the mind is informed of the contractions, and so enabled to regulate or guide them. The Sensations act chiefly on this description of muscles likewise, but partly also on muscles the nerves of which give no such distinct inti mation of their contractions, and which are uniformly and strictly involuntary; and the chief excitants of this last class of muscles in the Animal Economy are physical stimuli, ap plied to themselves and to their linino- mem bran es.
II. In regard to the vital power or property of Irritability, as exhibited in any of these ways, the following facts demand particular notice.
1. The minutest fibres, of which the mus cles, exhibiting this property, consist, or what have been called by some authors the primary filaments of muscular fibres, appear under the microscope to consist of rows of globules, or at least to be marked by transverse striw, at equal distances.
2. When contraction takes place, these fila ments, or rows of globules, are thrown into a zigzag form ; the angles being always at the same points on each contraction, and being generally obtuse, rarely and only on occasion of very forcible contraction, acute. At the points where these angles are formed, the fila ments are crossed, according to the observa tion of Prevost and Dumas, by nervous fibrils; but it is important to remernEer, that this last observation has been made only on muscles of voluntary motion, and on them only in cold-blooded animals, where they are somewhat translucent.
3. According to the best observations, not made on entire limbs or even entire muscles, which involve various fallacies, but on small portions of muscles, removed from living bodies, it appears that no alteration of the bulk of the filaments attends this alteration of their form, so that neither the size nor distance of the particles or globules appears to be changed, but merely their position in regard to one another.f.