292 MB. r. G. p.YESON^s OK THE [Mar. 20, 



of the bead of the fibula aud in most cases joins the outer head of 

 the gastrocnemius to help form the tendo AchiUis. In Ccelogenys 

 and Cavia cohaya, however, the muscle continued separate to its 

 insertion in the tuber calcis, while in Ceredon it was inserted into 

 the OS calcis by a round tendon from its inner portion, while the 

 outer portion blended with the tendo Achillis. 



Plantaris. — The plantaris rises just above the outer head of the 

 gastrocnemius ; it forms a muscular belly as large as, or larger than, 

 either one of the gastrocnemius. It soon contracts into a narrow 

 tendon, which winds round the inner side of the tendo Achillis, and 

 passes over the back of the tuber calcis to the sole, where it 

 spreads out into a broad fascia, which eventually splits into slips 

 for the four outer toes or as many as are present. 



Each of these sUps acts as a flexor perforatus, allowing the long 

 flexor tendons to pass through, and is then inserted in the same 

 way as the flexor sublimis in the fore limb. In Castor, where the 

 muscle is perhaps better developed than in any other Eodent, the 

 tendon divides into a superficial and a deep layer when it reaches 

 the commencement of the sole. The superficial layer is fibrous 

 and corresponds to the plantar fascia ; the deep layer develops 

 muscular fibres and doubtless represents the flexor brevis digitorura 

 of human anatomy. In many cases a loop is given ofE from the 

 deep surface of each tendon before it is perforated; this loop 

 embraces the long flexor tendon as in the anterior extremity. 



Popliteus. — The popliteus always has the usual human attach- 

 ments, except that it is often inserted only into the inner border 

 of the upper third of the tibia instead of into the posterior surface 

 right across. 



Flexor Lowjus Hallucis {Flexor Fibularis). — The long flexors of 

 Eodents have been so thoroughly described by Dobson^ that it would 

 be waste of space to do more than refer the reader to his Monograph. 

 I have repeated his dissections in many animals aud can confirm the 

 accuracy of his descriptions. The additional animals that I have 

 dissected fully bear out his point that among the Hystricomorpha 

 the flexor fibularis is joined in the sole by the flexor tibialis and is 

 inserted into the terminal phalanges of all the toes. In the 

 Sciuromorpha, on the other hand, the flexor fibularis goes to all 

 the toes without being joined by the flexor tibialis in the sole. In 

 Aulacodus and Dcm/procta no fibres were continued to the inner- 

 most toe from the flexor flbularis. 



Flexor Longus Digitorum {Flexor Tibialis). — This muscle, as 

 Dobson ^ points out, rises from the back of the tibia and in the 

 Hystricomorpha joins the flexor fibularis in the sole, its fibres 

 being continued chiefly to the inner toes. In the Sciuromorpha 

 it does not join the flexor fibularis, but in Sciunis, Xerus, Spermo- 

 philus, and Arctomys is inserted into a sesamoid bone below the 

 internal cimeiform, from which some ill-marked fibrous tissue is 

 continued on to the hallux. In Castor it terminated in the inner 

 half of the double scaphoid. The only exception to this arrange- 

 * Journ. Anat, vol. xviii. p. 159. 



