In this tract no secretion is visible with the naked eye, but with the aid of the microscope I have found, in perfectly recent animals, mi nute globules of extreme tenuity and of va rious sizes, which had all the appearance of mucus oozing from the interstices of the epithe lial particles. It is impossible but that the cilia should move these globules along the sur face, and discharge them into the pharynx ; and it hardly admits of doubt that mucus, morbidly existing on the bronchial membrane, is gradu ally lifted up by these untiring agents to that region where it excites coughing, and is forcibly expelled by the rush of air. The patient is often conscious of its slow motion upwards, when it is in the form of a pellet and proceeds from an isolated spot. This is remarkably the case too in h:emoptysis, and also in that rare disease the bronchial polypus, where branched tubes of lymph are brought up in this manner.
This view of the use of cilia in the mucous system of the higher animals appears to me to merit much attention. I had intended to have considered it under a separate head, but it has been introduced here both in corroboration of the general position as to the nature of secre tion, and in illustration of the nature and extent of the special secretion from the ordinary mu cous membranes.
On the whole I think it may be concluded: 1. That every part of the mucous system, where epithelium exists, secretes.
2. That the secretion differs, in different re. gions, according to the vital properties of their epithelia; and that these vital properties are usually attended with appreciable varieties of structure. That corresponding varieties of che mical constitution coexist with these is highly probable, though only as yet proved in a few cases.
3. That mucus is the least peculiar of the secretions, yet by no means universal from the mucous membranes, but confined to tracts of comparatively limited superficial extent, chiefly the excretory channels of the glands.
In the preceding summary account of the structure, relations, and offices of the mucous system, I have not been able (without inter ruption to the course of the description) to refer sufficiently to the labours of those anato mists to whom we owe almost all our know ledge of the subject. This deficiency, of which I am very sensible, I shall endeavour in some degree to supply by a brief review of the re searches which have led to the more modern and general views on the subject. Passing over the imperfect descriptions of the ancients, we find that when the microscope first became an instrument of anatomical research, the scaly character of the cuticle was recognised by Mal pighi and Leeuwenhoeck ; and that the former of these great anatomists had a wonderfully clear insight, considering the period at which he lived, into the close relation that subsists between the glands, mucous membranes, and skin. The labours of the anatomists of the next age were spent with great success upon matters of detail, particularly on the distribution of the blood vessels, which Ruysch and Lieberkiihn particu larly illustrated ; and, by the general advance of knowledge, the way was being gradually prepared towards that more philosophical ar rangement of the tissues of the body, in con formity with their intimate texture and con nexions, of which the first example is to be found in the work of Bonn,* already alluded to. He here traces, with great accuracy, the continuity of the skin and mucous membranes at the different orifices of the body, and he clearly recognises their close structural rela tion, considering the mucous membranes to be productions of the skin. To our countryman,
Dr. Carmichael we are indebted for the first application of this arrangement to the purposes of pathological classification, and Pinel soon after followed in the same track.t But a new era dates from the remarkable works of I3ichat,§ in which he delineated the structure, vital and other properties, and the relations of the different tissues of the body, and arranged them on a basis, which, though faulty in some of its details, has received no essential modification in its principles since his time, and entitles him to the praise of rare genius and sagacity. He seems to have clearly perceived the true connexion that exists between the skin, mucous membranes, and glands, al though lie failed to carry out his views into the subdivisions of his system, where he was still fettered by the crude notions of his predeces sors. One of the most remarkable features in his work, bearing on the present subject, is the analogy he draws between the epidermis of the skin and the mucus of mucous membranes, an analogy which he discovered with the eye of the mind, which has been since often rejected, but which can now be shewn to be real by the eye of sense.
Most writers of eminence since the time of Bichat have adopted the principal part of his views, and some have advanced further towards a full recognition of the homology of the skin, membranes, and glands, among whom must be mentioned, in particular, J. Midler, whose classical work on the glands,11 published in 1830, placed him at once in the foremost rank among the anatomists of our own day. Subsequently to that date, the improve ments in the construction of the microscope and the consequent employment of greater magnifying powers, have added an extraor dinary stimulus to anatomical and physiolo gical studies, and directed a host of inquirers into an almost unexplored field, from which the harvests already reaped give the most fa vourable earnest of future and rapid additions to the stores of knowledge. To the Germans is unequivocally due the merit of having far outstripped all other nations in the honourable path thus opened, and in no collateral path of inquiry which has been pursued to the same extent, has so much new, interesting, and im portant information of an accurate and satis factory character, been obtained, as in that which it has been my duty to treat of in the present article. The names of Purkinje, Va lentin, IIenle, and Schwann deserve primary honour in this place, and to these may be added those of Ehrenberg, Treviranus, R. Wagner, Boehm, Wasmann, Gruby, and Gerber. Among the French anatomists, MM. Turpin, Mandl, and DoniM have contributed much, and our own countrymen have not been be hind. Dr. Sharpey, Dr. Sprott Boyd, Dr. Todd, Mr. Nasmyth, Dr. Barry, and Mr. Toynbee, are all distinguished in this field of research.* In the present article I have endeavoured to combine with all the authentic information I could obtain from these sources, the results to which I have been brought by a two-years' study of the anatomical characters of the whole mucous system. So rapid, however, are the daily advances of knowledge, that it is possible much has been omitted which is already in some shape before the public, and, on the other hand, that a few years may greatly modify the general views that are here set forth. As the anatomical details, however, have been all sub stantiated by my own observations, except where otherwise stated, I am enabled to speak with more confidence of their correctness.