Of the nerves of distribution of the nerves through muscular structures has always been a subject of great interest with those who looked to this line of inquiry for some clue to the explanation, either of that wonderful active connexion subsisting between them, or of the nature of the contractile act itself. But though the anatomical results ac cruing from this inquiry are of a highly satis factory kind, considered in themselves alone, or as to what may be its nature, it would, per haps, in the present state of knowledge, be hardly warrantable even to speculate.
d. Of the distribution if the striped and unstriped fibre in the body.—The striped fibre is met with in all the voluntary muscles, and in a few involuntary, as the constrictors of tire: pharynx and the muscular coat of the oeso phagus, and the heart. In the oesophagus it appears to be mingled with the other variety to a somewhat uncertain extent. In some speci mens from the human subject I have been un able to detect any striped fibres in the lower half of that tube, either in the circular or longitudinal layer, but in other examples they have been traceable to within an inch of the stomach. Among the lower animals consider able differences occur, as has been well pointed out by Mr. Gulliver,* who observes that in general " the muscular fibre of animal life (striped) extended further towards the stomach in the outer than in the inner layer of the oeso phageal muscular sheath." In several animals this gentleman found the striped fibres even on the stomach (as in the Rabbit, Lepus cuniculus, Linn., the Sheep, Ovis cries, Desm., the Sloth Bear, Ursus labiutus, Blainv.) while in many others he met with them to within a very short distance of that cavity. Dr. Todd has also shewn them to me on the glandular pouch at the cardiac extremity of the stomach in the Dormouse. It is still unknown in what manner the two varieties of fibre are arranged at this point of junction, some supposing that they are simply intermixed, others that they pass into one another by imperceptible gradations. The former of these views is that which appears most consonant with my own observations. Mr. Skey considerst that the fibres of the heart " possess a somewhat compound character of texture," and this opinion seems highly pro bable. They possess, it is true, the transverse stripes indicative of an arrangement of particles in parallel series, but there is frequently a want of that uniformity and precision in this appear ance which so remarkably characterize voluntary muscle. The cross lines are apt to be broken and interrupted, and are sometimes difficult to discover at all. This condition is well repre sented by Mr. Skey in fig.5 of his second plate. In some of the smaller and lower animals the particles never form lr,insverse stripes. These fibres, as explained by him, are smaller than the average diameter of the voluntary muscles of the same subject by two-thirds, and in most parts of the heart they are not aggregated in parallel sets, but twine and change their relative position. Striped fibres have been found in the iris, in the small muscles of the ear, and in those muscular fasciculi that surround the ure thra immediately in front of the prostate. They are also found in the sphincters of the anus and vagina.
The unstriped fibre is met with in the ali mentary canal from the middle of the oeso phagus to the rectum, and constitutes the double layer investing that tube. It also forms the muscular coat of the bladdei: and that of the uterus. The dartos owes its contractility to the presence of fibres of this variety, which, in con sequence of the abundant admixture of areolar tissue, has not hitherto been clearly recognized; but it may be detected by the addition of acetic acid, which, by bringing into view the peculiar corpuscles it contains, distinguishes it from both the white and yellow fibrous elements of the areolar tissue. But even without this chemical test, to which some may object, it is possible to discover this form of muscular fibre in the dartos, by the characteristic appearances which have been already attributed to it. Since satis
fying myself of the real existence of this fibre in the dartos, I have on many occasions de tected a very decisive peristaltic action advan cing from one side of the scrotum to the other, and continued for a considerable period, yet of a kind which it was impossible to refer, with any degree of probability, to the cremaster. In one case particularly, occurring in the prac tice of Mr. Fergusson, where the tunics dartos was much hypertrophied, in connection with an old stricture of the urethra, we observed this peristaltic contraction of a very vigorous de scription. The fibres which have been de scribed as peculiar to the dartos seem to be nothing more than a certain modification of the areolar tissue in that region. In the corpora cavernosa penis of the horse there is a large quantity of this kind of fibre, as may be ascer tained by microscopic examination, although Professor Miller* seems indisposed to consider it really muscular. He states that " viewed in the microscope these fibres do not present any resemblance to muscular fibres," but my own examinations of them have not confirmed this statement. They appear to me to have all the characteristics of this variety, and by acetic acid are seen to contain a great number of cor puscles. Moreover they appear to consist che mically of fibrine. Professor Muller has failed in exciting contraction in them in the living horse, but this is not a conclusive fact as to their nature, for it may be and probably is the case that they do not act unless stretched by the erection of the organ. In the quiescent condi tion of the part they may be considered to be in a contracted state, like the muscular coat of an empty intestine, and so would naturally appear to be unaffected by the stimulus of galvanism. The erection of the penis seems with great probability to he attributable to pressure exercised on the superficial veins of the organ by a continuation of a structure ana logous to the dartos, and certainly containing the unstriped muscle, which is continued over the base of the penis under the skin. The erection of the nipple also occurs, on any me chanical irritation, with a motion so very much resembling the peristaltic action of muscular fibres, that I have little doubt such would be found, constituting a layer, under the skin in that region. And it may be matter of inquiry how far the general contractility of the skin is de pendent on a diffusion of this tissue, in small quantities, throughout its areolar structure. The excretory ducts of all the larger glands appear to possess a covering of fibre belonging to this Variety. Such is the case with the ductus cho iedochus in Birds, and probably in Mammals, and in the ureters and vasa deferentia. The bronchial tubes may be mentioned under this head as the best marked example of this ar rangement. The trachealis muscle consists evi dently and entirely of the unstriped fibres, and the same may be traced down the bronchial ramifications as fir as the air-cells themselves, though not into them. The distinctive charac ters of this form of muscle may here be une quivocally discerned, and if anatomists had been better acquainted with them, there would not have been room for those disputes regarding the muscularity of the bronchial tubes which have so long attracted the interest of practical physicians. Recently, indeed, there has been added to the satisfactory evidence of anatomy the well proved fact that these fibres may be excited to contraction by the galvanic stimulus.* in the case of other glands it is still unknown how far the muscular coat invests the ramifica tions of the duct ; it is most likely that it gra dually ceases a short way within the organ, and at least it seems clear that no portion of the secreting membrane itself is ever in vested by it.