Cat_Dissect__s_ - EXTERNAL ANATOMY Generally speaking,...

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EXTERNAL ANATOMY Generally speaking, orders of mammals are recog- nized rather easily by their external appearance; therefore, the external anatomy of a cat will be quite different from that of a rat and vice versa. The exter- nal features which separate mammals into orders are such characteristics as the number of digits on the feet, method of walking or other locomotion, charac- teristics of the teeth, etc. Mammals have two unique external characteristics which distinguish them from all other vertebrates: (1) All mammals have hair at some time during their development, and (2) all female mammals possess mammary glands with external openings for nourish- ing the young. The cat's head consists of an anterio-ventral facial region and a posterio-dorsal cranial portion. The lips around the mouth are well-developed, and the upper lip is usually cleft in the center by a groove called the philtrum. The external nares are situated on a naked nose. The eyes have upper and lower lids and a greatly reduced nictitating membrane called the plica semi- lunaris. The ears possess a long, flexible external fold called the pinna which directs sound waves into the external auditory meatus. The cat and many other mammals possess special, long sensory hairs on the face which are called vibrissae. The trunk is divided into an anterior thorax and a posterior abdomen. Teats or nipples (the external openings of the mammary glands) are located on the ventral surface of the trunk. The number and loca- tion of the nipples vary in different mammals. There are usually eight, four on each side, in the cat. The mammary glands are modified skin glands which produce milk. Most mammals have separate urogenital and anal openings. The anal opening is located at the base of the tail dorsal to the urogenital orifice. In males, the urogenital structures consist of the penis and a double pouch, the scrotum, containing the testes. The tail of mammals, including the cat, is usually well developed, but it is reduced in rabbits and absent in humans. The horny claws on the digits are an additional epidermal derivative. Other mammals have nails, hoofs, or horns which are also derived from the epidermis. In the cat, the claws are re- tractable. The position of the hand and foot in locomotion varies considerably, but it is usually constant within a single order of mammals. In the cat the gait is called digitigrad'e, meaning that the animals walk on the digits with the remainder of the hand and foot elevated. Humans are. plantigrade, i.e., they walk on the entire sole of the foot: Horses and cattle walk on hoofs (modified claws) and are unguligrade. THE SKIN The principal covering of the mammalian body is hair. Hair is exclusively a mammalian structure and is present in all mammals at some time during their life.
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This note was uploaded on 03/31/2008 for the course BIOL 129 taught by Professor Sachshowardgeor during the Spring '08 term at Penn State.

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Cat_Dissect__s_ - EXTERNAL ANATOMY Generally speaking,...

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