viii.] THE MUSCLES. 343 



The obturator externus is also a constant muscle both as 

 to existence and position, being found even in the Frog. A 

 peculiar sort of additional external obturator (but with an 

 anomalous internal origin) may exist, as in the Hyrax, where 

 a third obturator springs from the inner surface of the ischium, 

 close to its junction with the pubes, and which, passing 

 through the obturator foramen, is inserted into the trochan- 

 teric fossa in common with the obturator externus (Fig. 308). 



O.frr 



FIG. 308. RIGHT SIDE OF PELVIS OF HYRAX, seen in front. 



/, iliacus ; Pin, psoas magnus ; P, pectineus ; Oter, obturator tertius ; Oe, obtu- 

 rator externus (cut and reflected) : Qf, qu^dratus femoris. 



Tensor vagina femoris. This may be altogether wanting, 

 as in the true Opossums. It may, on the contrary, be very 

 largely developed, as in the Seals (where it arises from the 

 panniculus carnosus and external oblique) and in the Horse. 

 It may be more or less blended with the gluteus maximus, 

 as in Agouti. 



Sartoritis. This muscle is generally present in Mammals, 

 though it may be altogether wanting, as in Bats. Its origin 

 and insertion may both differ considerably from what we find 

 to exist in man. Thus, in the Agouti it arises from the 

 ilio-pectineal eminence and the pubic symphysis. It may 

 have two heads (one on each side of the tendon of the 

 psoas magnus), as in the Horse. It may be confounded 

 above with the tendon of the long extensor of the thigh, as 

 in Ruminants. It may arise, as in the Three-toed Sloth, 

 from the lower part of the aponeurosis of the external 

 oblique, and then divide into two parts one of these being 

 inserted into the femur, the other again subdividing and 



