lO ANATOMY OF THE RABBIT 



to Striated muscle, for most all striated muscles are subject 

 to reflex action and others, such as those which have to do 

 with the act of swallowing, the intercostals and diaphragm, 

 which function in breathing, function both voluntarily and 

 involuntarily. 



The structural unit of striated muscle tissue is the cylindric 

 or prismatic fiber or multinucleated cell, which is about o.oi 

 mm. in diameter and seldom exceeds 4-5 cm. in length; however, 

 fibers over 12 cm. long have been found in the sartorius muscle 

 (Addison). Usually the greatest diameter is near the middle 

 of the fiber and grows less toward either end. Each fiber, or 

 cell is composed of cytoplasm, called sarcous substance, 

 within which are several nuclei and which is enveloped by a thin, 

 often indistinguishable, sheath, the sarcolemma. The sarcous 

 substance consists of two principal parts, the passive sarco- 

 plasm and the highly speciahzed contractile fibrHlae. The 

 cross striations of alternating dark and light bands depend 

 upon the arrangement and constitution of the long, threadlike 

 contractile fibrillae, and, in the final analysis, are probably only 

 regions of different densities; the less dense, or optically lighter, 

 are singly refracting regions and are called isotropic while the 

 denser, or optically darker, region is double refracting and is 

 called anisotropic (Addison). 



A skeletal muscle is composed of a great number of multi- 

 nucleated cells or fibers, which are held together by an areolar 

 connective tissue sheath, the endomysium, and thus constitute 

 the small secondary bundle, or fasciculus. Varying numbers 

 of secondary bundles are enveloped by a connettive tissue 

 sheath, the perimysium, and thus constitute the primary 

 bundle. Primary bundles are in turn grouped together and 

 enveloped with a connective sheath, the epimysium, to form 

 the skeletal muscle. The epimysium (muscle sheath) is con- 

 nected with the perimysium (sheath of primary bundles) and the 

 endomysium and sarcolemma, so that there is a continuous 

 connection between the sheath enveloping the entire muscle, 

 the primary and secondary bundles and the fibers. However, 



