As mentioned at the beginning of the chapter, much of the more immediate environment is experienced by the animal through its skin.
This sense, referred to as exteroception, includes sensations such as touch, pressure, pain, heat, and cold; touch is a light stimulus such as is produced by a fly on the haircoat, and pressure is a stronger and deeper stimulus such as a horse feels from a saddle or girth.
The receptors responsible for the detection of these stimuli vary considerably in structure. Unfortunately, because many intermediate forms exist, it is difficult to classify them and assign clear-cut functions to each kind. The simple classification given here is probably adequate for the purpose of this book.The sensory receptors of the skin can be divided according to strictly anatomic criteria into free nerve endings and nerve endings that bear terminal corpuscles. The free nerve endings are tufts formed by the branches of nerve fibers that terminate either in fine points or in button-like swellings; they are found principally in the epidermis, and their purpose is to detect noxious, or painful, stimuli (Fig. 9.33/1). The corpuscular endings fall into three kinds: bulbous, lamellar, and meniscoid. The bulbous corpuscles, which are encapsulated terminal tufts of nerve fibers found in the dermis, are thought to respond to heat or cold (Fig. 9.33/2). The lamellar corpuscles are large (23 mm) and each consists of many concentric lamellae (flattened cells) surrounding the distal end of an afferent nerve fiber; they are found in the subcutis and are thought to be pressure receptors (Fig. 9.33/3). Meniscoid corpuscles are small cup-shaped disks (menisci) at the ends of nerve fibers with which they contact "tactile" cells; they are found, usually encapsulated, both in the papillary layer of the dermis and free in the adjacent epidermis and are thought to be touch receptors (Fig. 9.33/4).
FIG. 9.33
Sensory nerve endings of the skin, schematic. 1, Free nerve endings (pain); 2, bulbous
corpuscles (heat or cold); 3, lamellar corpuscles (vibration); 4, meniscoid nerve endings (touch).
A special cutaneous sense is mediated by the tactile hairs. They are long hairs that protrude from the head and are substantially thicker than the hairs forming the haircoat. The cat, s whiskers are good examples, but all domestic mammals have them, principally in association with the muzzle and eyes. The roots of tactile hairs are surrounded by blood sinuses, the walls of which are densely populated with afferent nerve endings (see Fig. 10.12). When the tips of the tactile hairs are touched, the movement is amplified by fluid movement in the sinuses, exciting the sensory nerve fibers, which transmit this message to the central nervous system (see also p. 346).
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