Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovsky 2008

Special Part
Sense Organs, organa sensuum
Organ of Touch

The Organ of Touch is the Skin, cutis, which covers the entire surface of The Human Body and transitions into mucous membranes at natural body openings. The skin is a vast receptor field with an area of approximately 1.6 m².

It is densely packed with nerve endings, which enables it to function as the organ of touch. However, The Significance of the skin in body METABOLISM and vital activity is not limited to this. The skin forms the General Integument of the body, integumentum commune. Its function is to protect the body from injury, infection, and dehydration. Through the skin's Blood capillaries, body Temperature is regulated. It also serves as a depot for energy reserves and a site for metabolic processes, excretion, and Respiration.

The skin consists of a superficial layer—the epidermis, epidermis, represented by a Cytology/practical/35.html">Stratified squamous keratinized epithelium—and the dermis (true skin), dermis (corium), formed by dense Fibrous Connective Tissue (Fig. 233). Located within the dermis are Hair roots and bulbs, radices et bulbi pili, sebaceous and Sweat Glands, glandulae sebaceae et sudoriferae, arrector pili Muscles, mm. arrectores pilorum, and pigment Cells (Fig. 233). Blood and Lymphatic vessels are also situated there.

The dermis comprises two layers: the superficial papillary layer, stratum papillare, and the deep reticular layer, stratum reticulare. The papillary layer forms numerous projections directed toward the epidermis—papillae, papillae—which merge to form skin ridges, cristae cutis. These ridges are most pronounced on the skin of the palms, soles, and fingertips, where their patterns are characterized by individuality and permanence. Four MAIN TYPES OF patterns are distinguished on the fingertips: arches (I), loops (II), whorls (III), and double loops (IV) (Fig. 234 A, B). This anatomical feature is used in forensic investigations as a means of personal identification (dactyloscopy).

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Fig. 233. Structure OF THE Skin (after Kiss and Szentágothai)

In certain areas of the skin, skin furrows, sulci cutis, are formed by all layers of the skin. The direction of these furrows is determined by the orientation of the dermal elastic fibers. During surgical Procedures, the edges of incisions made along the elastic fibers easily adapt, leaving delicate scars. Conversely, the edges of incisions made across the elastic fibers are difficult to adapt, resulting in coarse (often keloid) scars. Therefore, from a cosmetic standpoint, it is preferable to make incisions along the skin furrows or the so-called skin lines (Langer's lines) (Fig. 135).

The skin is connected to the underlying fascia and periosteum by means of loose connective tissue—the subcutaneous layer (hypodermis), tela subcutanea. The amount of adipocytes within it varies depending on a person's sex, age, constitution, and nutritional status. In the forehead and Nose regions, the subcutaneous base is relatively thin, measuring 1–2 mm. This layer is particularly well-developed on the abdomen, buttocks, and soles. Pathological processes in the subcutaneous tissue can lead to edema development.

Fig. 234 A. Skin ridges, cristae cutis, and skin furrows, sulci cutis (imprint of the palmar surface of the right hand)

Fig. 234 B. Main types of fingertip patterns: I - arch; II - loop; III - whorl; IV - double loop

Fig. 235. Orientation of skin lines on the human body

Among the keratinous appendages of the skin are hair, pili, and Nails, ungues. The hair ROOT, radix pili, along with the bulb, as previously noted, is located in the dermis, while the shaft, scapus, projects freely above the skin surface.

The nail is a keratin plate covering the dorsal surface of the distal phalanx. The free margin, margo liber, lateral margins, margo lateralis, and nail root, radix unguis, are distinguished.

The area of skin upon which the nail body, corpus unguis, rests is called the bed, lectulus, while the area underlying the nail root is the matrix, matrix. The skin folds that embrace the nail on the sides and at its root are called the lateral nail fold, vallum laterale, and the posterior nail fold, vallum posterius.

As already mentioned, the skin Functions as an organ of touch thanks to numerous nerves and nerve endings. They are especially abundant on the palms, fingers, face, and external genitalia. Located within the skin are nerve endings that provide tactile sensitivity (touch and pressure sensation), pain, and thermal sensitivity. These receptors initiate the Pathways of the exteroceptive analyzer. The detailed structure of cutaneous nerve endings is described in Histology textbooks.

Knowledge of the features of skin innervation is of major clinical significance for topical Diagnostics of various diseases (radiculopathy, herpes zoster, nerve injuries), for selecting physical therapy Methods, and for correctly performing local anesthesia. Figures 236 A and B illustrate the sensory zones of segmental (or radicular) skin innervation, which appear as bands. On the trunk, these zones are arranged approximately horizontally, whereas on the limbs, they acquire an oblique-vertical orientation. This is due to the fact that, in phylogeny, the zones of segmental innervation formed long before humans assumed an upright posture. In a person with arms lowered, all radicular zones run parallel. This pattern reflects the correspondence between neuromeres (from which Spinal Cord segments developed) and dermatomes. It should also be borne in mind that adjacent radicular zones overlap in such a way that each strip of skin receives sensory innervation not only from its corresponding segment but also from the segments immediately above and below it. Consequently, the selective lesion of a single nerve root does not result in a distinct area of total sensory loss.

The innervation Zones of the thoracic segments correspond to the innervation zones of the intercostal nerves, because the latter are segmentally arranged. As for other Regions of the body, such a direct correspondence does not exist. This is because, due to The formation of plexuses (cervical, brachial, lumbosacral, and coccygeal), the nerve fibers of a single root enter into the composition of several nerves, while each nerve, as a rule, is formed by fibers from multiple roots. Therefore, the area of sensory loss resulting from peripheral nerve damage will differ from that caused by a lesion of the corresponding posterior roots of the spinal cord.

The summary table lists the areas of skin innervated by their respective nerves.

Fig. 236 A. Diagram of segmental (radicular) skin innervation zones (anterior view)

Fig. 236 B. Diagram of segmental (radicular) skin innervation zones (posterior view)

Skin of the HEAD and Neck

Summary Table

Skin of the Trunk

Skin of the Upper Limb

Skin of the Lower Limb

Knowledge of the so-called Zakharyin–Head zones (Fig. 237) is also of great clinical significance. These are specific areas of skin where pain sensations, as well as pain and temperature hyperesthesia, frequently occur during visceral diseases. Each internal organ has its own projection zone on the skin. The size of the projection zone depends on the state of the sympathetic and parasympathetic Innervation of the organ. Zakharyin–Head zones are of great diagnostic importance, since the onset of pain, hyperesthesia, and changes in electrical potential levels are observed not only during the acute period of organ disease, but also during remission. Furthermore, changes in electrical potential in the projection zones are recorded even before the first signs of internal organ disease appear. The formation of such zones is caused by the spread of irritation originating from the affected internal organ to the spinal centers. From there, excitation is projected as a Pain Sensation (or hyperesthesia) onto those skin areas innervated by the roots corresponding to these centers. Thus, in lung diseases, the projection of pain occurs in the area innervated by the III–IV cervical and II–V thoracic roots; Heart — III–V cervical and I–VIII thoracic (predominantly on the left); Esophagus — V, and sometimes VI–VIII thoracic; Stomach AND Pancreas — VII–IX thoracic; Liver — III–IV cervical and VIII–X thoracic (on the right); Gallbladder — VIII–IX thoracic; Kidney — X, and sometimes IX–XII thoracic and I lumbar; Ureter — XI–XII thoracic and I lumbar; Testes — XI–XII thoracic; Urinary Bladder — XI–XII thoracic, I lumbar, and III–IV sacral; Prostate Gland — X–XI thoracic and I–III sacral; Ovary — X thoracic; fallopian tubes — XI–XII thoracic; Uterus — X–XII thoracic, I lumbar, and I–IV sacral

* Russian therapist G. A. Zakharyin (1883) and English neurologist H. Head (1898) independently described these dermatomal areas in visceral diseases for the first time.

Fig. 237. Diagram of pain sensation areas (Zakharyin–Head zones) in visceral diseases