PERONEAL MUSCLES IN BIRDS. 1067 



fibula below the insertiorx of the biceps tendon and from the 

 adjacent surface of the tibia ; the fusiform belly gives rise to a 

 short tendon which frequently, but not invariably, traverses a 

 fibrous bridge and then flattens out to the normal insei-tion to 

 the proximal end of the tarsus- metatarsus. 



The minor difi'erences which are to be found, but too irre- 

 gularly to be correlated with size, habit, or systematic position, 

 relate to the relative sizes of the two muscles. Sometimes the 

 P. longus is rather feebler with little deep origin and s relatively 

 narrow superficial oi'igin ; the P. brevis, on the other hand, 

 becoming very thick and strong. The muscular bellies of both 

 muscles may be shoi-ter and their tendons longer. 



In only two of the Passeriform birds examined did I find 

 notable differeiices. In Memira superha, the inclusion of which 

 amongst the Passeriformes I doubt on other grounds, the anchor 

 of the P. longus was much shorter and rather wider than usually 

 occurs in the group, and the P. brevis was relatively considerably 

 weaker. In Hirundo 7-ustica the P. longus had the usual broad 

 superficial oiigin but very little deep or fascial oiigin, and the 

 slip to III was absent. The P. brevis was of the usual Passerine 

 type and relatively weaker than in the Swifts. 



The Coraciomorphine birds present maiiy different phases of 

 the degeneration of the longus and of the increasing importance 

 of the brevis, and in this res2:)ect are in marked contrast with the 

 other Legions into which Di-. Gadow has arranged the Orders of 

 birds. 



Summary and Conclusions. 



The Peroneus longus muscle shows every gradation from 

 elaboi-ate structure and apparently important function to com- 

 plete absence, and the facts fall into a coherent picture if we 

 suppose that we have to deal with loss of parts originally present. 

 The presence and absence of origin from the fascife of the neigh- 

 bouring muscles are the most irregular and may well be interpreted 

 as in direct relation to function. If the muscle be large and 

 important, it overgrows its bounds and comes in organic contact 

 with the adjacent fascise ; if it is smaller and less important it 

 remains isolated. When the muscle as a whole appears to be in 

 process of degeneration, this may proceed in two ways. Occasion- 

 ally, but rarely, the deep origin begins to disappear first, and the 

 superficial oiigin gradually narrows until it becomes only a thin 

 superficial tendon. More often the process starts with the 

 degeneration of the superficial origin, and this is often accom- 

 panied by strengthening and lengthening of the deep origin, 

 until finally nothing but the deep origin is left. This next 

 gradually shortens until the whole muscle disappears. Of the 

 tendons of insertion, the slip to III goes first and the anchor 

 persists. There are many cases in which the slip to III has been 

 lost, the anchor remaining as the only insertion, but I have found 

 no instance in which the anchor has disappeared leaving only the 



