504 



MUCOUS MEMBRANE. 



application in accordance with that necessity 

 for precision of thought and expression which 

 characterizes modern science. The exposition 

 contained in the article Mucus will render it 

 superfluous for me to define its present accept- 

 ation. It is denied by Dr. Gruby that the 

 viscid form of mucus is a normal secretion 

 from any membrane whatever, and he considers 

 its existence as a certain mark of diseased ac- 

 tion. This view, if less absolute, would be in 

 a great measure correct, since there is no doubt 

 that in a state of perfect health most mucous 

 surfaces are wholly unprovided with any pro- 

 tection of this kind. If the nasal cavities, the 

 trachea or bronchia, the intestinal or urino- 

 genital tracts, be examined in a healthy animal 

 killed for the purpose, we may search in vain 

 for any slimy covering, such as they are com- 

 monly imagined to possess. But in a state of 

 disease, each of these surfaces will secrete great 

 quantities ; and it is not a little remarkable, that, 

 even when healthy, if moistened and allowed to 

 undergo slight putrefaction, they will become 

 coated with a viscid fluid, having the physical 

 characters of mucus. Yet the slirny fluid of the 

 mouth cannot with propriety be considered 

 abnormal. The true saliva is not viscid, as it 

 escapes from the ducts of the glands into the 

 cavity of the mouth : it probably becomes so 

 by dissolving the substance derived from the 

 scales of epithelium lining the mouth, as they 

 advance to the surface and flatten. The fluid 

 of ranula is not merely the accumulation of a 

 natural secretion, but seems gradually to ac- 

 quire its great viscidity by receiving the debris 

 of the epithelium lining the excreting channels, 

 and by the partial reabsorption of its aqueous 

 portion. 



In the intestinal canal, however, although 

 there is no viscid mucus naturally present, yet 

 there is a large amount of" inspissated mucus" 

 being continually separated from the villi and 

 follicles of Lieberkuhn. This mucus, as already 

 mentioned, is nothing more than the debris of 

 epithelial particles. 



But chemists have detected, in most of the 

 secretions, a small proportion of a substance 

 nearly allied to rnucns, and probably a form of 

 it. There is good reason to believe this to be 

 the product of the membrane lining the ex- 

 cretory passages, and to represent the old epi- 

 thelium of that membrane. Where the secre- 

 tion of the gland is fluid and in considerable 

 quantity, it seems to be sufficient to convey 

 away this debris from the surface which it tra- 

 verses on its way out of the system, as in the 

 salivary and allied glands, the liver, kidney, 

 &c. But where, from the absence of this 

 means of carrying off the debris of the epithe- 

 lium, it might be supposed to be liable to 

 accumulate and clog the surface, cilia are de- 

 veloped ; of which the best example is fur- 

 nished by the respiratory tract, the nasal ca- 

 vities, and the tympana. That this is the 

 great office of these wonderful organs upon 

 these extensive surfaces appears to be proved 

 by the fact that the currents they produce are 

 uniformly towards an outlet, lienle has ob- 

 served this in several parts, and I have ascer- 



tained it by experiment in the case of the tra-* 

 cheal and bronchial membrane. 



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 haemoptysis, 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, 



