In the genetic disease called ichthyosis, the skin is rough, brown, and scaly because the uppermost layer cannot peel off as easily as it should. What layer of the skin is affected in this disorder?

The marked structure is the xiphoid process, the smallest and most inferior portion of the sternum. It is a thin, elongated structure located below the body of the sternum and serves as an important anatomical landmark in the thorax. During early life, the xiphoid process is composed primarily of hyaline cartilage and gradually ossifies with age. It provides attachment sites for the diaphragm, rectus abdominis, and transversus thoracis muscles, contributing to respiration, trunk movement, and stabilization of the anterior thoracic wall.

A. Manubrium: The manubrium is the broad, superior portion of the sternum that articulates with the clavicles and the first pair of ribs. It contains the jugular notch and clavicular notches, making it an important landmark for identifying thoracic structures. Its primary function is to provide attachment and support for the pectoral girdle and upper ribs. Unlike the xiphoid process, it is located at the superior end of the sternum rather than the inferior tip.

B. Body of sternum: The body of the sternum is the largest and longest portion of the sternum, situated between the manubrium and xiphoid process. It articulates with the costal cartilages of ribs 2 through 7 and contributes significantly to protection of the heart, lungs, and major thoracic vessels. Compared with the xiphoid process, it is much larger and forms the central portion of the anterior thoracic cage.

C. Xiphoid process: The xiphoid process is the small, inferior segment of the sternum located immediately below the sternal body. It serves as an attachment point for the diaphragm, rectus abdominis, and transversus thoracis muscles, making it important in respiration and trunk stabilization. Clinically, it is used as a landmark during cardiopulmonary resuscitation (CPR), as incorrect hand placement over the xiphoid can result in fracture and injury to underlying organs. Since the highlighted structure is the inferior terminal part of the sternum, it corresponds to the xiphoid process.

D. Costal cartilage: Costal cartilage consists of bars of hyaline cartilage that connect the anterior ends of the ribs to the sternum. These cartilages provide elasticity to the thoracic cage, allowing expansion and recoil during breathing. They are paired structures extending laterally from the sternum rather than a single midline structure. Unlike the xiphoid process, costal cartilage is not a component of the sternum itself but serves as a connection between ribs and sternum.

The human nervous system relies on distinct receptor types to convert physical and chemical stimuli into electrical signals for interpretation by the central nervous system. Each receptor type is tuned to a specific form of energy, allowing the body to detect light, chemical changes, tissue injury, and temperature variations. This classification is fundamental to understanding sensation, perception, and homeostatic regulation.

• Light energy → Photoreceptors: Photoreceptors are specialized sensory cells located in the retina of the eye that detect light energy and convert it into electrical impulses. They include rods, which are responsible for low-light vision, and cones, which detect color and detail in bright light. These receptors are essential for vision and visual processing. Without photoreceptors, the nervous system would be unable to interpret light stimuli from the environment.

• Changes in concentrations of chemicals → Chemoreceptors: Chemoreceptors detect chemical changes in the internal and external environment, including oxygen, carbon dioxide, pH levels, and taste or smell substances. They are found in structures such as the carotid bodies, aortic bodies, taste buds, and olfactory epithelium. These receptors play a crucial role in regulating respiration, metabolism, and sensory perception. They help maintain homeostasis by detecting chemical imbalances in the body.

• Any factor that causes tissue damage → Nociceptors: Nociceptors are pain receptors that respond to harmful or potentially harmful stimuli such as mechanical injury, extreme temperature, or chemical irritation. They are found throughout the skin, muscles, joints, and internal organs. Activation of nociceptors sends pain signals to the central nervous system, alerting the body to injury. This protective mechanism helps prevent further tissue damage.

• Changes in heat energy → Thermoreceptors: Thermoreceptors detect changes in temperature, including both heat and cold. They are located primarily in the skin and hypothalamus and help regulate body temperature by initiating behavioral and physiological responses. These receptors allow the body to maintain thermal homeostasis by sensing environmental and internal temperature changes. Dysfunction of thermoreceptors can impair temperature regulation and increase risk of heat-related or cold-related injury.