OCR Text |
Show 1894.] MYOLOGY OP RODENTS. 293 inent that I have seen was in Pteromys oral, in which the flexor tibialis divided into two slips, one of which had the usual Sciuro-morphine insertion below the internal cuneiform, while the other joined the tendon of the flexor fibularis. Possibly this was an individual variation foreshadowing the arrangement in the Hystricomorpha. Tibialis Posticus.-The description of the tibialis posticus is iucluded in that of the "Long Flexors of Rodents" by Dobson1. In Castor it is inserted into an extra bone on the inner side of the internal cuneiform. Lumbricales.-The number of the lumbricales seems to depend on the number of toes; thus all the Sciuromorpha and the Hystricomorpha possessing five toes, such as Myopotamus, have four lumbricales. Ccelogenys, although it has five toes, has only three lumbricales. Animals having three toes usually only possess two lumbricales, e. g. Dasyprocta, Cavia cobaya, and Ceredon rupestris. In Dolichotis Beddard only found one 2, but in another specimen which I had the opportunity of looking at there were two. Muscles of the Foot. The accessorius is absent in the Dipodidae and Caviidae, but present in the other animals examined, including the Sciuromorpha. It rises from the outer surface of the calcaueum, usually from the anterior part, and is inserted into the plantar surface of the flexor tendon just before it divides for the toes. The angle which it forms with the flexor fibularis is a very opeu one, about 45°, but in Hystrix it must be about 70° or 80°. W h e n the foot is well developed there are two interossei to each metatarsal bone. W h e n the hallux is well developed, as in Myopotamus, the abductor hallucis rises from the sustentaculum tali, or from the scaphoid, as in Octodon, but when it is not developed the muscle is absent. In the Sciuromorpha the abductor minimi digiti often rises from the calcaneum as well as from the base of the fifth metatarsal; in this case the part between the calcaneum and the metatarsal will form an abductor ossis metatarsi quinti. On the plantar surface of the interossei there are frequently found two muscles rising from the deep cartilage of the sole which forms the sheath of the peroneus longus tendon; from this they run forwards, diverging from one another like the limbs of a V. The inner of these is in some of the Sciuromorpha inserted into the outer sesamoid boue under the head of the metatarsal bone of the hallux, forming an adductor hallucis, but more often, as in Myopotamus, Octodon, Hystrix, and Ccelogenys, it is attached to a corresponding situation on the second toe forming an adductor secundi digiti. The adductor minimi digiti (the outer of the two muscles) is attached to the inner sesamoid bone of the little toe. These two muscles are wanting in the Dipodidae and the Caviidae, 1 Journ. Anat. vol. xvii. p. 159. 2 P. Z. S. 1891, p. 236. PROC. ZOOL. Soc-1894, No. XX. 20 |