Section I. or TBB CUTANEOUS SYSTEM IN GENERAL. The mombrnnes of the cutaneous Byslem are gene- rally disposed in the form of ramilying canaLs, which, wilb the exertion of the lining of the alimentary pas- Mgc, ultimately end in Mind extremities, or culis de 8ac. This form does not immediately strike the mind, because the mucous membranes are usually orrnnged in a Tcry complicated manner ; but if any one of their pro-' cesses be carefully traced, it becomes apparent ; thus, if- the hepatic duct be followed into the substance of the' liver, it will be perceived that its lining membrane at length terminates in a closed extremity. The external integument has, strictly speaking, a similar configura* tion in many parts of its extent ; this may be seen in the cul de sac of the skin which encloses the end of each of the fingers and t<]es. <Callout type="important" title="Important">The continuity between the skin and mucous membranes is crucial for understanding their functions.</Callout> The two great divisions of the cutaneous system are continuous with each other at all the natural openings of the body, viz. at the mouth, the nostrils, the aper- tiu-e of the eyelids, and at the commencement of the' urinary and genital passages, The skin and the mucous membrane, although they arc in such direct communi- cation with each other, are dissimilar in their external' appearance ; and this circumstance has had considerable' influence in inducing many anatomists to doubt the identity of their composition. But this is a very in- sufficient objection to support the conclusion that has been drawn from it ; for, if similarity in appearance were required as the proof of the identity of structure, the most serious errors would be committed. If such reasoning were admitted, no one, for example, could believe, what is certainly true, that the delicate arachnoid' of the ventricles and the coarse peritoneum of the loins,'' are both specimens belonging to the same class of membranes. It is necessary, then, to seek for other sources of information in order to determine the point in question. Tlie first may be derived from an examination of the embryo in the early period of its existence, at which lime there is so little difference between the external and the internal integument, that they are distinguished from each other with difficulty. The perfect continuity of the skin at the external apertures of the body with the internal membranes, in another proof which supports the opinion of these structures being identical. At the lips, at the nontriU, at the eyelids, at the anus, at the meatus unnarius, at the labia pudcndi, we see the skin endually changed into the mucous membrane. It is true that around some of these openings a slight line of demarcation exists, as at the tarsal edge of the eyelidt^ and at the lips ; and a similar appearance may be more evidently observed in some of the internal parts of the body, which are contiguous with each other but distinct' in function ; as for example, where the cuticuiar lining of the ffisophagutt joins with Uic villous tunic of thfl' stomach, and, in a less degree, in the connexion between the inner membrane of the ^tiginR and the 'fl5cu- lar coat of tlte utenu. But these distinctions do not extend to the structure of the two divisions of the cutaneouN system, and therefore it cannot, with any truth, be asserted, that one is not perfectly continuous with the other. The organization of the animals which are placed lowest in the scale, displnys, in a striking manner, the n^somblancc of Uie extt-rnal and internal coverings of the hudy ; in these creatures there are none of those diversities of appearance that have given rise to so much uncertainty in the human subject ; the skin and the mucous membrane arc, in fact, so completely identical, that one may, with impunity, be substituted for the other ; so that if the polypus be turned inside-out, the new internal surface acquires the power of digestion, and the new external stffftice that of protecting the body. The effects of certain diseases, as for example, the prolapsus uteri and the prolapsus ani, occasionally show that the mucous coat of the internal organs may become so altered by exposure to the air, as to assume in time some of the characters of skin. We may then, I think, safely conclude from the preceding observations, that the differences which exist between the two great divisions of the cutaneous system, relate principally, though not entirely, to their external appearance. These diversities not only characterize the skin and the mucous membrane, but also their individual parts, as may be seen by comparing the skin in the palm with that of the dorsum of the hand, or the skin of the scalp with that of the fingers, The differences are still more apparent in the mucous membranes ; of the truth of this statement we may satisfy ourselves by comparing the lining of the frontal sinuses with that of the Stomach, or even with the proper SduielderiaD membrane. In the places where the skin and mucous membranes are continuous with each other, the former becomes more thin and smooth, and its tissue more delicate. According to Bichjit, the limits between them are indicated by a reddish line ; this may be traced on the lips, but at the other openings the change in appearance is so gradual, that no distinct separation can be observed. This disposition of the cutaneous system is such, that it possesses two surfaces ; one of which is free, and the other adherent. In the skin, the former is placed externally, and the latter internally. In the mucous membranes, it is said that the arrangement is reversed ; but strictly speaking, in the stomach, the intestines, &c., the free surface is still situated externally as regards the passage of foreign substances. The adherent surface is united to the substance of the body by the subcutaneous and the submucous cellular tissue, the quantity and density of which are liable to great variation. The internal portion of the cutaneous system is generally in immediate relation with the involuntary or hollow muscles ; but the external portion is, in most parts of the body, separated from the muscles of volition by a strong fascia or aponeurosis. The free surface possesses a much more intricate and important organization than that which adheres. His complicated structure is provided for the defence of the cutaneous texture against the action of foreign bodies, and for the performance of its extremely numerous and diversified functions. In all parts of this surface, numerous small depressions are furnished, which have been named by some writers according to their situation and the fluid they secrete, simple glands, sebaceous glands, and mucous glands ; by other anatomists they are called follicles, lacuitie, cryptse, S;c. These minute bodies exist in every part of the cutaneous system ; they vary in size, and also in shape, some being round, and others oblong or infundibular. Each of them is formed by a fold of the entire membrane, having the form of a cul de sac, the mouth of which is the duct of the follicle and conveys its secretion to the free surface. The openings of the follicles are very evident in some places, as on the skin of the nose, where they appear like dark grey spots ; in the mucous membrane, on the contrary, the microscope is required to bring them into view. The number and disposition of these bodies is subject to great variation ; in some parts, although they are very numerous, they are distinct from each other ; this is the arrangement in the skin and in some of the mucous membranes ; in other parts they are confused together, as in the formation of the tonsils, &c. 'The follicles are supplied by many vessels which are derived from those of the integuments. The free surface, especially in the mucous structures, presents a great number of wrinkles and folds, which are of a very different character according to the part in which they are placed ; some of them are permanent ; whilst many among them exist or not, as the organ containing them is empty or distended. <Callout type="tip" title="Tip">Understanding the structure of these glands can help in identifying potential health issues.</Callout> H <Callout type="risk" title="Risk">Misunderstanding the continuity between skin and mucous membranes could lead to improper treatment of wounds.</Callout> 178 OBOAHIZATIOM OR THE Sectiok 11. ORGAKIZATfON or TBB CITTANBOOS STaTtV. This system consists of certain layers superposed each other ; they are so different in their structure, that they might almost be considered as so many separate systems, if it were not preferable to regard them only as many parts of the same system. The amount of these layers varies in the skin and mucous membranes ; in the former, three are generally enumerated, the der* mis, the rete mucosum of Malpigfai, and the epidermis ; the latter consist essentially of one layer only, the mu- cous corium or dermis. Meckel describes, as a fourth lamina, the textus papillaris, which is, apparently, only a portion of the dermoid texture. The dermis, which is the most deep>seatcd and solid layer, is united in all its extent to the subjacent struo tuies, and thus gives support to the other parts of the CDtaneouN xystem. It is whitish, soft,, and elastic, and b cOnipowd o( a tibrous layer of cellular tissue, vhich varies in its thickness and density. It rcccix'cs a great number of vessels and nerves which arc placed on its external surface, where they form a delicate structure consisting of the minute divisions of arteries, veins, lymphatics, and nerves ; from this important fabric numerous minute bodies project, which are called nervous or vascular papilla.'. The relc or corpus mucofium, which is only met with in the skin, consists of a very soft and delicate substance interposed between the dermis and the epidermis. It receives the termination of a great number of sanguineous capillaries, and lodges the commencement of many lymphatics. It is the seat of the colouring matter, and, according to some anfttoinists, of those homy processes which are attached to some parts of the cutaneous organ. The epidennis forms the most external part of the skin und of some of the mucous membranes ; it is whitish, solid, and apparently destitute of vessels and nerves. It is considered by many writers as being an inoi^nic concretion, but for reasons which will after- wards be advanced, I am inclined to admit it as one of the organiied solids. It is essential to the protection of the body against the injurious influence of external agents, which immediately cause irritation in any part of the surface where it may have been accidentally de- tached. It is equally necessary for the preservation of animals and vegetables, forming in both, as it has been well observed, a fine but essential barrier between life and destruction. The epidermis cannot be traced in the majority of the mucous membranes, in which the constant secretion of a mucous fluid appears to supply its place. <Callout type="important" title="Important">The epidermis plays a critical role in protecting against external harm.</Callout> The chemical composition of the cutaneous system, notwithstanding the attention which has been paid to it, is not yet perfectly understood. The best English and French chemists describe the cutis as consisting chiefly of jelly. Dr. Bostock thinks, that the fibrous part of the skin, which forms its proper substance or basis, is composed of albumen, and that it has, intermixed with H, a quantity of matter of a different chemical nature, which he supposes to be a compound of jelly and mucus. The properties of the cutaneous system, like those of membranous structures in general, are almost entirely of a physical character. The colour of the skin and mucouA membranes is different ; and in both, but especially in the former, it is liable to many variations. The colour depends, in part, on that of the blood, and in part on a colouring matter which is secreted from that fluid. The seat of this substance in the skin, is the corpus mucosum ; its appearance varies, not only according to the different varieties of mankind, but also in individuals of the same race, country, or even family. In the mucous membranes there is more uniformity, the colour generally being light yellow-, often tinged with red or brown. The density of this system, which is greater in the skin than in the mucous membrane, is very considerable ; it exceeds that of the cellular tissue, but it is less than that of the ligamentous organs. <Callout type="important" title="Important">Understanding the coloration and density differences can aid in diagnosing issues.</Callout> The extensibility and flexibility of the cutaneous membranes are very great ; in the skin, these properties allow of the varied movements of the body being performed without any risk of laceration ; in the mucous membranes they admit of the change in size which is so frequently required in the hollow viscera, as in the stomach, the intestines, the urinary bladder, &c. ; the alteration in the volume of these organs partly depends, however, on the unfolding of the wrinkles which exist on their internal surface when they are collapsed. The contractility, or, as it has been called, the tone of these structures, is very much developed. It is occasionally exerted with great rapidity, as when the mouth suddenly contracts on the air or food which it contains, Generally, however, the contraction takes place slowly, and often to a great extent ; thus the lining membrane of the uterus, after delivery, gradually retracts to its original size, and again in the permanent artificial anus, the inferior portion of the intestine is so much diminished in its calibre, that the colon is occasionally reduced to the size of the finger. The contractility of the skin, although considerable, is inferior to that of the mucous membrane; it is known, for example, that in the removal of a large tumour from the surface of the body, if a sufficient quantity of the integument be not taken away the skin is afterwards loose and wrinkled, and a similar appearance is observed on the lower part of the abdomen in women who have borne several children. Some anatomists have admitted, that the integuments are irritable, or, in other words, that they contract on the application of a stimulus ; this opinion is, I believe, altogether erroneous ; for, as far as I have had an opportunity of observing, the skin and the mucous membranes when they are irritated in a living animal, exhibit no proofs of contraction independent of that which is caused by the action of the subjacent muscles. The sensibility which these parts possess, is their most remarkable property, and enables them to fulfil their important offices in the economy. In the skin it is very manifest, for it is in this organ that the sense of touch is exercised. Many parts of the mucous tissue are endowed with a high degree of sensibility; thus two of the special sensations, viz. smelling and tasting, reside in the commencement of the extended mass which lines the digestive and pulmonary organs. Again, the surface of the glaas penis, and of the urethra in the male, and of the vagina in the female, possesses an exquisite sensibility which is greatly excited during coitus. In most parts of the mucous membranes there is, however, only that obscure kind of sensibility which enables them to receive the different impressions of the foreign substances that are brought into contact with them ; it is necessary to add, that these impressions are never perceived by the mind, in the healthy state of the organs, although in disease the natural stimulants, as the food in the stomach and the urine in the bladder, will excite the most painful sensations. The functions of the cutaneous system, which are extremely complicated and diversified, are very important, and immediately connected with the support of life. The most universal and essential use of the external integument, is to protect the body against the decomposing and destructive influence of chemical agents, which instantly exert their power wherever this defen- giv coveriing is removed. In man, and in a less degree in the higher classes of animals, the skin is an exquisite sensitive organ, which assists to establish their relations with the external world, by receiving the impressions of the innumerable bodies that surround them. The mucous membrane is equally a defence to the internal organs ; it constitutes a barrier through which all foreign substances must pass in order to enter into, and to form part of, the body ; and at the same time no substance which has for a certain period constituted a part of the body, can pass from it, but through the medium of the mucous membrane, or of the skin. 133 The free surface of the tegumentary system is a most extensive organ of secretion and absorption. There are two distinct secretions performed by the skin ; one is derived from the sebaceous glands or follicles, and the other from the minute termination of the arteries. The first secretion defends the skin from the influence of the foreign bodies by which it is surrounded ; the second purifies the blood, and also assists in regulating the animal temperature. <Callout type="tip" title="Tip">Understanding these secretions can help in managing skin health.</Callout> The mucous membrane has likewise two secretions ; one consisting of a mucus, which is provided for the protection of these structures against the contact of foreign substances ; and the other, composed of a vapoury halitus, somewhat similar to the perspiration of the skin, has probably the same use as that fluid, as far as purifying the blood is concerned. <Callout type="important" title="Important">Understanding both types of secretions in mucous membranes can aid in diagnosing issues.</Callout> The functions of the cutaneous system are extremely complicated and diversified ; they are very important, and immediately connected with the support of life. The most universal and essential use of the external integument is to protect the body against the decomposing and destructive influence of chemical agents, which instantly exert their power wherever this defensive covering is removed. In man, and in a less degree in the higher classes of animals, the skin is an exquisite sensitive organ, which assists to establish their relations with the external world, by receiving the impressions of the innumerable bodies that surround them. The mucous membrane is equally a defence to the internal organs ; it constitutes a barrier through which all foreign substances must pass in order to enter into, and to form part of, the body ; and at the same time no substance which has for a certain period constituted a part of the body can pass from it, but through the medium of the mucous membrane, or of the skin. The free surface of the tegumentary system is a most extensive organ of secretion and absorption. There are two distinct secretions performed by the skin ; one is derived from the sebaceous glands or follicles, and the other from the minute termination of the arteries. The first secretion defends the skin from the influence of the foreign bodies by which it is surrounded ; the second purifies the blood, and also assists in regulating the animal temperature. The mucous membrane has likewise two secretions ; one consisting of a mucus, which is provided for the protection of these structures against the contact of foreign substances ; and the other, composed of a vapoury halitus, somewhat similar to the perspiration of the skin, has probably the same use as that fluid, as far as purifying the blood is concerned.
Key Takeaways
- The cutaneous and mucous systems are continuous at natural body openings.
- Und