Section IV. DEVELOPMENT OF THE CUTANEOUS SYSTEM. The embryo appears, in the beginning, to be entirely composed of the skin and mucous membrane. A communication exists from an early period after conception between the umbilical cord and the interior of the alimentary canal; many writers suppose that there is a similar connection between the allantois and the urinary bladder. At this time, the anterior part of the body and also the intestinal tube are open, so that there is no complete continuity between the internal and external membranes. Afterwards, the walls of the intestine become closed to form the canal; and the skin begins to close on itself, with its lateral halves coalescing on the median line, which retains several indications of the original division even at the end of life, such as the raphé of the scrotum and perineum. The mucous surface of the intestines is simpler in the fetus than it is subsequently due to the absence of convolutions and valvulae conniventes; these folds begin to appear towards the end of pregnancy. The organization is also defective, owing to fewer vessels and nerves, and thinner skin and mucous membrane compared to after birth. Sebaceous follicles are observed to be very much developed by the sixth month, covering the fetus with a caseous and greasy substance defending it from liquor amnii contact. A quantity of viscous fluid is found in the intestines which changes color from whitish-mucous to dark-green; this is called meconium. Erroneous ideas have prevailed amongst some writers concerning these two substances, but there can be no doubt that they are secreted by sebaceous and mucous follicles respectively. The cutaneous system does not exhibit remarkable changes in the advance of life excepting that the skin loses elasticity and becomes loose and wrinkled on its surface; also, the activity of its secreting and absorbing powers is diminished.
PART SECOND. OF THE SKIN. The skin forms the outer covering of the body, connecting different parts together and defending it from external agents. All animals are provided with such a covering although properties and appearance vary greatly. In humans, the skin is extremely complex in organization; it consists of distinct layers each exercising peculiar functions: cuticle or epidermis, rete mucosum (doubted by many), and dermis or corium. The existence of the rete mucosum as a proper membrane has been doubted by many high authorities although there are not wanting others who have adopted Malpighi's opinion; this consideration is deferred till described.
The skin has an external surface with furrows, pores (doubtful), and numerous hairs. The internal or adherent surface of the skin is totally different in appearance from the outer; it is looser in texture and gradually confounded with subjacent cellular tissue. This tissue is generally lax allowing varied body movements but being more condensed fixing the skin closely as on scalp, median line of abdomen.
The epidermis, called also cuticle or scarf skin, is the most external layer of the skin; it is transparent, thin and delicate except in places where it has been compressed. It covers the exterior of the body and reflects into all openings formed on its surface lining passages leading from them. Thus it lines eyelids, anterior part of eyeball, external auditory canal, nostrils, mouth, tympanum, pharynx; continues along esophagus to stomach, trachea to lungs; also extends into anus, vagina, urethra.
The epidermis is the most extended membrane of the skin covering all parts exposed to air or foreign bodies. This was observed by Haller who said he knew no part of human body which could be exposed to air with impunity except cuticle and enamel of teeth. The external surface presents numerous wrinkles, most proper to structure of skin; these have been well described by Chevalier.
The internal surface presents an appearance precisely the reverse of outer and also of outer surface of true skin to which it is applied. When cuticle raised from layer beneath, most hairs are extracted with it but some still remain attached to dermis if separation accomplished by putrefaction a number of very delicate transparent colorless filaments observed which admit extension before giving way.
Many investigations made in order to be certain if the cuticle possesses any pores for transmission of perspiration and various substances absorbed by skin. Leeuwenhoek thought he had seen them; more recently Bichat endeavored to prove their existence stating that if portion of epidermis examined in favorable light immense number of pores observed crossing its thickness in oblique direction. Most anatomists confess inability to detect these passages.
Opinions respecting formation of cuticle uncertain and contradictory. It has been supposed by some writers that it is originally formed by hardening of surface of skin which accomplished before birth by pressure of liquor amnii and afterwards by atmospheric pressure; this hypothesis certainly erroneous for as compression exerted upon body before and after birth uniform in all parts epidermis according to explanation ought be equal thickness throughout but find contrary even in fetus some portions thicker than others according to office they destined fulfill. Many anatomists regard cuticle as inorganic concretion exuded on surface of skin constantly renewed destroyed exterior proportion produced interior notwithstanding insensible no blood-vessels absorbents or nerves satisfactorily traced it still appear sufficient proofs that like all other parts living body organized substance.
Key Takeaways
- The skin and mucous membrane are the primary components of an embryo in its early stages.
- Sebaceous follicles develop significantly by the sixth month, providing protection to the fetus.
- The epidermis is crucial for protecting the body from external agents and has a complex structure.
Practical Tips
- Understanding skin development can help in recognizing signs of fetal health during pregnancy.
- Recognize that the skin's protective function remains vital throughout life, even as it ages and loses elasticity.
Warnings & Risks
- Do not assume the cuticle is an inorganic layer; it is a living tissue with its own vascular system.
- Be cautious about interpreting historical anatomical descriptions without modern scientific context.
Modern Application
While this chapter focuses on embryonic skin development, understanding these principles can be crucial for medical professionals and survivalists alike. Knowledge of how the skin forms and functions from an early stage aids in recognizing potential health issues during pregnancy and post-birth. Additionally, it highlights the importance of proper hygiene and protection against environmental factors throughout life.
Frequently Asked Questions
Q: What is the significance of sebaceous follicle development by the sixth month?
Sebaceous follicles develop significantly by the sixth month, providing a protective layer to the fetus that defends it from contact with liquor amnii.
Q: Why are anatomists uncertain about the existence of pores in the epidermis?
Anatomists are uncertain due to conflicting evidence; some believe they exist based on microscopic observations, while others cannot detect them even under powerful magnification.
Q: What is the role of the cuticle according to historical anatomical descriptions?
The cuticle or epidermis forms the outermost layer of skin and serves as a protective barrier against external agents. It also lines various body openings, such as eyelids and nostrils.