RAT, A REPRESENTATIVE MAMMAL 121 



The structure of muscle is discussed in more detail elsewhere in 

 the book but a brief statement concerning it may be appropriate 

 here. Each skeletal muscle is composed of a large number of fibers 

 or cells as they are called. These are bound together by connective 

 tissue (fascia). An individual fiber is slender and enclosed in an 

 elastic membrane called the sarcolemma. Within, is the general mass 

 of protoplasm, which is composed quite largely of the longitudinally 

 disposed myofibrils. These are surrounded by the more fluid portion 

 of the sarcoplasm. Each fibril in certain states of contraction shows 

 alternate dense and light areas along its length. Adjacent fibrils 

 across the entire fiber have these areas at approximately the same 

 level, thus giving the appearance of cross-striation through the en- 

 tire fiber. This is the basis for speaking of this tissue as striated 

 muscle. Each fiber possesses numerous nuclei, making it multi- 

 nucleate. Smooth muscle of the visceral organs does not possess the 

 dense and light areas on the fibrils of the cells. 



In detailed study of the muscles they are usually taken by 

 regions or groups according to location. It is assumed that the 

 student will have more detailed descriptions of the muscles in labor- 

 atory directions if they are studied there. Reference should be 

 made to Greene, ''Anatomy of the Rat," Tr. Am. Philosophical Soc, 

 Vol. XXVII, 1935, for more detailed descriptions and illustrations. 



Dig^estive System 



This system is in the form of a tube with various outgrowths and 

 extends from the mouth to the anus of the animal. It serves pri- 

 marily in digestion of complex food material, converting it into a 

 soluble form which may be absorbed by the blood and lymph. The 

 entire system may be divided into a number of parts as follows : 

 buccal cavity (mouth), pharynx, esophagus, stomach, small intestine, 

 large intestine, salivary glands, liver, and pancreas. 



The most anterior portion, the buccal cavity, is bounded dorsally 

 and ventrally by the upper and lower lips. The space between the 

 teeth and the inner surface of the lips and cheeks is called the 

 vestibule. The teeth have been described previously with the skull. 

 The tongue is a well-developed organ lying in the floor of the mouth, 

 but it does not have the usual frenulum, or band of connective tissue, 

 extending from its ventral side to the floor of the mouth. There are 

 papillae (horny processes) over the anterior and lateral portions of 

 the dorsal surface. There are taste buds in the walls of some of 



