Human Anatomy (with Fundamentals of Dynamic and Sports Morphology) - Ivanitsky M. F. 2008

The Doctrine of the Sense Organs
General Integument of the Body

The Skin forms the external covering of the entire body. It not only protects the body against various external influences but also plays a role in thermoregulation, excretes Metabolic waste products, and perceives Temperature, pain, tactile, and partly proprioceptive stimuli, which is why it is discussed in the chapter dedicated to the Sense Organs.

The skin consists of three layers (Fig. 140): the outer layer, or epidermis; the skin proper (dermis); and the subcutaneous base containing fat deposits. The latter form the subcutaneous adipose layer.

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Fig. 140. Skin (cross-section):

1 - epidermis; 2 - dermis (skin proper); 3 - subcutaneous adipose layer; 4 - fascia; 5 - Muscle; 6 - vein; 7 - artery; 8 - nerve; 9 - nerve ending; 10 - sweat gland; 11 - sweat gland duct; 12 - arrector pili muscle; 13 - sebaceous gland; 14 - Hair; 15 - papillary layer of the skin (after Maxwell)

The epidermis is somewhat transparent and thin, with a thickness ranging from only 0.3 to 4 mm. In areas subject to significant mechanical stress, such as the soles of the feet, it is considerably thicker than, say, on the eyelids or the nipples.

The epidermis is covered by a stratified squamous keratinizing epithelium, the Cells of which gradually slough off. They are replaced by cells from the deep layer, which continuously divide and multiply.

The dermis (skin proper), 0.5–5.0 mm thick, consists of two layers: the superficial papillary layer, featuring numerous papillae projecting toward the epidermis, and the deeper reticular layer, containing A large number of Collagen, elastic, and reticular fibers. The papillary layer contains Blood and Lymphatic vessels as well as nerves. Therefore, in cuts or other injuries where the papillary layer is affected, bleeding occurs and pain is felt. Smooth muscle cells, occasionally gathered into small bundles, are also found in this layer. The presence of numerous elastic fibers in the dermis imparts its elasticity. With age, the Elastic properties of the skin diminish, and it becomes more flaccid.

The subcutaneous adipose layer is a stratum of adipose Connective Tissue located beneath the dermis. In an adult, its weight reaches 10–15 kg. The largest fat deposits are typically observed in the region of the anterior abdominal wall, the Mammary Glands, and the pelvis. In women, the subcutaneous adipose layer is better developed than in men, especially in the pelvic and thigh regions.

A sedentary lifestyle and excessively fatty foods contribute to an increase in fat deposition, which is harmful to the body because it increases the load on the Muscles and Heart during movement, and also causes a higher position of the Diaphragm, preventing its normal descent during inhalation. In adults, fat deposition usually increases simultaneously in both the subcutaneous layer and the Internal Organs. During the growth period, no such parallelism exists. Despite the relatively large development of subcutaneous adipose tissue in children (especially newborns), fat is not usually deposited in the internal organs at the same time.

As a result of intensive muscular training, The amount of subcutaneous adipose tissue can decrease significantly.

The skin possesses a wide array of appendages. These include hairs—with bundles of smooth Muscles Attached to them and Sebaceous Glands opening into the hair follicle (see Fig. 140)—Nails, and Sweat Glands.

In a hair, one can distinguish the part located within the skin—the ROOT—and the free part—the shaft, which ends in a pointed tip. At the end of the root is the hair follicle, through which the hair grows. A cross-section reveals that the hair consists of three layers: the medulla located in the center, the hair cortex, and the outer layer, the cuticle.

Hair color depends on the pigment located in the hair cortex, as well as on the air content within the hair and The properties of the cuticle. Graying of the hair is caused by the accumulation of air within it and a decrease in pigment content.

Hairs are positioned somewhat obliquely relative to the outer surface of the skin (see Fig. 140). On the side of the obtuse angle formed by the hair and the skin surface, the arrector pili muscle, consisting of Cytology/cytology/32.html">Smooth Muscle tissue, is attached. When this muscle contracts, the skin acquires a rough appearance (so-called goosebumps), and the hair becomes perpendicular to the outer surface of the skin. Muscle contraction also exerts pressure on the sebaceous gland associated with the hair's excretory duct, which facilitates the secretion of its sebum.

The function of hair boils down to reducing skin heat loss, increasing the evaporation surface for secreted sweat, and providing a protective role.

Nails appear as somewhat convex quadrilateral plates, the free edge of which faces the free end of the finger, while the opposite edge, as well as the two lateral edges, are covered by the skin. The nail is situated in a groove and is surrounded by the nail fold. The connective tissue nail bed is fused directly with the periosteum of the phalanx. Nail growth begins at its root, which is distinguished by its softness and bluish tint. Changes in nail growth occur when the root is damaged. Nails protect the distal Phalanges of the fingers from mechanical impact.

Sweat glands are a type of simple tubular glands (Fig. 140) whose secretory portion is coiled into a glomerulus. Sweat glands produce a liquid secretion, sweat, The excretion of which rids the body of metabolic waste products. The evaporation of sweat from the outer surface of the skin involves the absorption of body heat and helps lower body temperature during overheating. Sweat glands are present over almost the entire surface of the skin; they are particularly numerous on the skin of the palms, up to 1,000 per square centimeter. The total amount of sweat can be quite substantial—more than a liter per day.

The so-called apocrine sweat Glands of the skin are characterized by the shedding of PARTS OF THE cellular protoplasm into the lumen of the gland during secretion. These glands are located in the axillary region, the skin of the nipple, and the Perineum.

The skin features grooves and folds that can be permanent or temporary, depending on Muscle Development, fat deposition, etc. In addition to grooves involving all layers of the skin, there are permanent grooves and folds formed solely by the papillary layer of the Skin and its epidermis. The pattern of permanent line configurations (e.g., on the fingerprints and palms) does not change with age and can only be destroyed if the papillary layer of that skin area is destroyed. Every individual has distinct and unique features in the pattern of skin lines on the fingertips, palms, and soles (dermatoglyphics).

As is well known, skin darkens or tans under METABOLISM/18.html">The Influence of direct or reflected sunlight (ultraviolet rays), as well as artificial sources of ultraviolet light. The appearance of a tan is explained by the accumulation of the pigment melanin in the Cells of the deep epidermal layer. Tanning is a kind of protective reaction of the body to ultraviolet radiation. Given that the degree of tanning is related to the amount of Hemoglobin in the blood, a tan serves to a certain extent as an indirect indicator of a person's physical condition. There is an opinion that people who tan quickly and deeply are more physically resilient and better resist infections than those who tan poorly or are entirely incapable of tanning.

The Blood Vessels of the skin are located in its dermis and subcutaneous layers. These vessels form a fine capillary network around the skin papillae, as well as around hair follicles and sebaceous glands. Nerves terminate in the skin, forming specialized sensory receptor apparatuses. Additionally, skin nerves supply fibers to the hair muscles (sympathetic), glands, and blood vessels.

The skin contains several types of sensory nerve endings; these include encapsulated (non-free) nerve endings (lamellar corpuscles, bulbous corpuscles, tactile corpuscles, and tactile discs), as well as free nerve endings.

It is believed that the sensation of pain is transmitted by free nerve endings located in the epidermis and the papillary layer of the skin. The sensation of Touch is perceived by tactile corpuscles and tactile discs. The former are located in the papillary layer of the dermis, and the latter in the stratum basale (germinativum) of the epidermis. The sensation of pressure is associated with the presence of lamellar corpuscles in the skin lying within the adipose tissue. The sensation of warmth is perceived by Ruffini endings (or warm corpuscles) located in the lower part of the dermis, while the sensation of cold is perceived by end bulbs located closer to the epidermis.

From the receptors for tactile, pain, and thermal sensitivity of the trunk and limb skin, the spinothalamic pathway is formed, through which transformed stimuli reach the cortex of the postcentral gyrus. From the receptor cells of the scalp, dura mater, conjunctiva, oral and nasal mucosa, Paranasal Sinuses, and Teeth, the sensory portion of the Trigeminal nerve is formed, running via its nuclei in the Rhomboid fossa to the thalamus, and then to the region of the postcentral gyrus cortex. When movements need to be executed based on generated visual sensations, information can pass from the postcentral gyrus region to the precentral gyrus region, followed by the propagation of efferent impulses to the motor apparatus. Transmission of information to the motor apparatus can also occur through connections between the postcentral gyrus and subcortical motor nuclei.