COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 131 



to the tibia and is called the ischio tibialis. From the anterolateral margin of 

 the girdle a muscle, partly concealed under the pubofemoralis externus, projects 

 forward, and its fibers become continuous with the ventral portions of the 

 myotomes. This is the posterior end of the rectus abdominis noted above. 

 Around the anus is a large mass, the anal gland. On removing this gland two 

 muscles are revealed on each side. The medial one is the ischiocaudalis , extend- 

 ing from the posterior margin of the ischium to the tail, and the lateral one is 

 the pyriform, its anterior end fastened to the ischiotibialis, its posterior end pro- 

 ceeding into the tail. Follow these two muscles into the tail. Turn the animal 

 sidewise, bend the leg forward, and note that the pyriform splits near its anterior 

 end and gives off another muscle which passes to the femur. This muscle 

 is the femorocaudalis. The three muscles just mentioned are tail or caudal 

 muscles. 



Certain definitions may now be made. Each muscle is attached at its two 

 ends and more or less free in the middle. The attached end which is fixed and 

 immovable is named the origin; the attached end which moves when the muscle 

 contracts is the insertion. The best method of naming muscles is to combine the 

 origin and insertion into a compound word, the origin preceding. The names of 

 the majority of the muscles given above are of this kind, but muscles are also 

 frequently named from their shapes, positions, etc. In the case of many muscles 

 either end may serve as the origin or insertion, depending on which end is held 

 fixed; thus, the caudal muscles of Necturus will move the tail if the pelvic girdle 

 is fixed or will move the girdle if the tail remains stationary. The muscles of 

 the girdle already named serve chiefly to bend the limb toward the median 

 ventral line, a movement known as adduction. 



On the ventral surface of the thigh four muscles may be identified. These 

 are: the pubofemoralis internus, the most anterior one, originating on the anterior 

 rim of the acetabulum and inserted on the distal end of the femur; the pubotibialis , 

 next posterior to the preceding, extending from the acetabulum to the proximal 

 end of the tibia; next, the distal portion of the ischiotibialis, this part being some- 

 times designated the gracilis muscle; and most posteriorly, a slender muscle, the 

 femorofibularis, originating on the preceding muscle and inserted on the fibula. 

 Some of these muscles act to bend the shank toward the thigh, an act known as 

 flexion. 



On the dorsal side of the thigh in addition to some of the muscles already 

 mentioned, which appear also in dorsal view, there is found chiefly the ilioexten- 

 sorius, a broad band originating on the ilium (which may readily be found as a 

 curved bone between the myotomes) and inserted on the distal end of the femur 

 by a tendon (sheet of connective tissue) which also passes onto the shank. This 

 muscle abducts the thigh, that is, draws it dorsally, and straightens or extends the 

 shank. Between the ilioextensorius and the pubofemoralis internus, partly 

 concealed by the latter, is the iliofemoralis , extending from the ilium to the femur. 



