VIIL] THE MUSCLES. 



353 



third, fourth, and fifth digits respectively. They are muscles, 

 however, which are largely developed only in the Mammalian 

 class. 



The Peroneus tertius is, amongst Primates, exclusively 

 human. Sometimes (e.g. in some individual Guinea-pigs) the 

 tendon of the peroneus longus will, pass down in front of 

 the malleolus, and so simulate a peroneus tertius. It is 

 said to be present in the Wombat. 



Peroneus longus. This may be altogether wanting, as in 

 the Horse, and apparently in all below Mammals. Its 

 tendon may, as in Hyrax, pass outside the malleolus instead 

 of behind it. The muscle may take origin in part from the 

 femur (as in the Ox and Opossum), and be inserted into the 

 naviculare or second metatarsal. 



Peroneus brevis. This may be altogether absent in man's 

 class, as in the Agouti. It may be reinforced by peronei 

 muscles going to the fourth and fifth digits (and termed 

 peronei quarli et quinti digiti}, as in Lemur. These have 

 exceedingly slendr tendons. 



In the Rabbit, a muscle which springs from the front of 

 the tibia and passes behind the internal malleolus, going to 

 the extensor tendon of the second digit, has been called the 

 tibialis secundi digiti. This does not exist in man. 



The gastrocnemius is generally a double muscle, but there 

 may be only a single head and belly, as in the Echidna. The 

 Muscle may be very slender, and quite insignificant in size 



hen compared with the flexor longus digitorum, as is the 

 case in Lori's. The muscle may be divided into two lateral 

 portions, and become connected with quite other muscles, 

 as is the case in the Iguana and Chameleon. Thus, in the 

 Iguana the external head (which, contrary to the condition in 

 most Mammals, is larger than the internal) ends below in a 

 membrane which forms, as it were, perforated tendons for the 

 digits. This head may arise, as in the Chameleon, by a tendon 

 from the inter-articular cartilage. 



The internal head of the gastrocnemius of the Iguana is 

 closely connected with the insertion of the semi -membra - 

 nosus, and receives a tendon from the biceps. The internal 

 head may arise from the tibia only, as is the case in Parson's 

 Chameleon. 



This muscle (Fig. 314) may be absent or included in the 



semi-tendinosus (as in Menopoma), which in the sole becomes 



superficial to the flexor digitorum. The tendo Achillis, 



instead of being strongly inserted into the calcaneum, may 



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