170 MANUAL OF ANIMAL BIOLOGY 



muscles of the body wall and of the appendages. It is separated 

 into definite units, the muscles proper. For example, in the hind 

 limb of the Frog there are some eighteen separate and distinct 

 muscles which bring about the various movements of the leg. All 

 these muscles are attached to the bones by means of specialized 

 connective tissue elements known as tendons. In general, one 

 end of a muscle, known as the origin, is attached to an immovable 

 bony structure, while the opposite end, known as the insertion, 

 is attached to a movable bony structure. The gastrocnemius 

 muscle, which is a large muscle in the calf of the leg, may be taken 

 as an illustration of the origin and insertion of a muscle. It has a 

 double origin in, first, a tendon at the end of the thigh bone (femur) 

 just above the knee joint, and second, in a tendon attached to 

 another tendon from the triceps muscle higher up the leg. These 

 points of origin are relatively immovable. The insertion of this 

 muscle is in various foot bones. When it contracts it causes an 

 extension of the foot and also a flexing movement of the leg at 

 the knee joint. (W. f. 98.) 



A muscle that produces an extension, such as just noted in the 

 foot, is known as an extensor ; one that causes a flexion of the 

 part is known as a flexor. The gastrocnemius muscle produces 

 both these movements. There are a number of other types of 

 muscles classified according to the movement which they produce. 

 Among these are the adductor muscles which draw the limb in a 

 posterior direction toward the long axis of the body ; the adductor 

 muscles which draw the limb in an anterior direction toward the 

 long axis of the body ; the levator muscles which raise some part 

 of the body, such as the lower jaw, and the depressor muscles 

 which work in an opposite direction. 



A microscopic examination of striated muscle tissue shows that 

 it is always enclosed in a connective tissue sheath, known as the 

 perimysium, which contains blood vessels and nerves. The peri- 

 mysium continues beyond the end of a muscle as a tendon which 

 is usually attached to a bone. Sheets from the perimysium radiate 

 throughout the body of each muscle and separate it into a number 

 of muscle bundles, termed fasciculi. Each fasciculus contains a 

 great many individual muscle fibers which are, in turn, separated 

 from each other by a further continuation of the perimysium, 

 known as the endomysium. 



The muscle fibers, microscopic in size, and enclosed by a delicate 



