52 TliG Pliylogeny of tlie Crural Flexors 



ment for the oiitor surface of the plantaris for more than its proximal 

 half. The muscle also takes origin from the lateral surface of the lateral 

 sesamoid bone. The large muscle mass which results is intimately re- 

 lated to the plantaris, and in its upper part a thin sagittal aponeurotic 

 plate appears dividing the muscle into almost equal portions {Ge'- and 

 Ge'"). This sheet is continued all the way down the leg and below 

 receives the insertion of the muscle fibers, becoming the tendon. In its 

 upper part the muscle comes into contact with the gastrocnemius 

 medialis (Gi), an aponeurotic plate intervening between them, however, 

 and below, the tendons of the two muscles fuse completely to be inserted 

 into the tuberosity of the os ealcis, the tendon of the soleus also Joining 

 them shortly before their insertion, to form a tj'pical tendo Achillis.' 



In the mouse I cannot state the exact origin of the muscle (Figs. 10 

 and 12), but it has essentially the same structure as in the cat and has 

 similar relations with the gastrocnemius medialis and plantaris. 



Two other muscles are intimately associated with the gastrocnemius, 

 the one more especially at its origin and the other at its insertion. 

 These are the plantaris and the soleus. 



The plantaris, notwithstanding its variability in man, is of very con- 

 stant occurrence throughout the entire mammalian series, and has as 

 a rule a much greater development and a more important role than in 

 man. It is always closely associated at its origin with the gastrocnemius 

 lateralis and is inserted below into the plantar fascia (occasionally into 

 the OS ealcis) by which its action is transmitted to the digits. In the 

 three forms here under consideration it forms what may be regarded 

 as the medial anterior portion of the muscular mass formed by it and 

 the gastrocnemius lateralis. It arises in the opossum (Fig. 8, PI) from 

 the medial half of the posterior surface of a tendon which extends down- 

 wards from the lateral fabella, and in the cat, from the fabella and from 



* In sections through the tendo Achillis one sees to the medial side a dis- 

 tinct tendon, connected to the true tendo by thin fascia beneath which lies 

 the tendon of the plantaris as in a groove. This tendon might readily be 

 mistaken for that of the gastrocnemius medialis, but it is in reality a 

 thickening of the crural fascia and is quite independent of the muscle. It is 

 attached below to the os ealcis medially to the insertion of the tendo Achillis. 

 Upon the lateral border of the tendo a similar thickening of the crural fascia 

 occurs, but this fuses with the tendon of the gastrocnemius lateralis shortly 

 before it is joined by the soleus. These fascial thickenings have been described 

 for the dog by Ellenberger and Baum, gi, who trace them to the semi-mem- 

 branous and biceps muscles, but they do not seem to have been noted for 

 the cat, at least they are not mentioned in any of the works on that form 

 to which I have access at present. 



