. The biology of the amphibia. Amphibians. 322 THE BIOLOGY OF THE AMPHIBIA Heat and Cold Receptors.—Amphibia are sensitive to changes in temperature. This has been well shown by Wright (1914) and others who have recorded the temperatures at which different species of frogs appear from their winter quarters. It has been assumed that free nerve endings in the epidermis are the cold receptors (Plate, 1924). Morgan (1922) noted that the reaction time of frogs was longer to heat than to cold stimulation and concluded that the heat receptors lay deeper in the skin. By treatment with cocaine Morgan e
. The biology of the amphibia. Amphibians. 322 THE BIOLOGY OF THE AMPHIBIA Heat and Cold Receptors.—Amphibia are sensitive to changes in temperature. This has been well shown by Wright (1914) and others who have recorded the temperatures at which different species of frogs appear from their winter quarters. It has been assumed that free nerve endings in the epidermis are the cold receptors (Plate, 1924). Morgan (1922) noted that the reaction time of frogs was longer to heat than to cold stimulation and concluded that the heat receptors lay deeper in the skin. By treatment with cocaine Morgan eliminated the response to cold earlier than that to pain stimulation. The result suggests that the heat and cold receptors are different from pain receptors. After cocainizing the skin a re- sponse to acid and to pain per- sisted beyond the response to heat, and the response to heat and cold beyond that to touch. Therefore the receptors for acid, heat, cold, pain, and touch in the skin of the frog are probably different. Organs of Taste.—Although many fishes, such as the catfish, have taste buds over the entire outer surface of the body, these structures, and with them the sense of taste, became limited in the Amphibia to the mouth. The senses of smell and taste are closely allied physiologically but the organs are very different structurally. Taste buds are isolated groups of elongated cells widely distributed over the palate, jaws, and tongue (Fig. 115). The groups on the palate of urodele larvae are smaller than those of the tongue of adult frogs, and it has been suggested that these groups of sense organs in the frog may be tactile instead of gustatory organs. They consist of cylindrical as well as elongate cells (Niemack, 1893), both of which lack cilia, in contrast to most of the lining cells of the mouth. On the tongue of the frog the taste buds occupy the summits of fungiform papillae which are scattered among the filiform papillae and with them form the plushlike s
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