DR. J. MURIE ON THE ANATOMY OF THE WALRUS. 453 
The gastrocnemius is relatively narrower in the latter form. The plantaris does not 
cover so much of the belly of the gastrocnemius, and the two muscles are more nearly 
equal in volume, in Trichechus. The popliteus, with nearly the same attachments in 
both animals, lies quite at the back of the leg in the Walrus (woodcut 8, Po); neither 
does it touch the head of the tibia or internal lateral ligament, but glides obliquely 
across the head of the fibula. 
The remarkable semimembranosus and semitendinosus (Pl. LV. fig. 25, S.t) are 
each rather relatively broader, and the latter is inserted higher on the tibia. 
It may be noted of the sartorius (Pl. LV. fig. 25, Sa) that in the Walrus it is much 
broader than in the Eared Seal. With a fleshy origin from the anterior superior spine 
of the ilium, its fleshy mass expands inwards and covers the entire groin and front of 
thigh. Muscularly it is inserted into the patella; and fascia, derived outside that, is 
continued over the inside of the knee-joint. Thus the main difference from what 
obtains in Ofaria relates to width, and its covering or overlapping the insertions of the 
rectus femoris, vasti, pectineus, and adductor muscles. Its inner moiety is much 
thinner than the outer one. 
The extensor brevis digitorum in the Trichechidze is composed of three flat muscular 
slips, not, as in the Otariidee and Phocide, of two bellies. (a) The first or innermost 
fleshy portion in the former animals lies about the middle of the dorsum (woodcut 6, 
£.b.d"), and arises in front and to the fibular side of the astragalus. Almost at the 
head of the third metatarsal it becomes broadly tendinous, and spreads out on both 
sides of that bone, and is also fixed to the fibular side of the second metatarsal. 
(b) The second or middle portion (woodcut 6, £.0.d*) is similarly a flat muscular slip 
which has origin from thé fibrous tissues and interarticular fascia in front of the astra- 
galus, and partly from underneath the belly of the first portion. It joins the muscle 
next to be described at the head of the fourth metatarsal. (¢) The third or outer 
portion (woodcut 6, #.b.d*) arises as a fleshy strip from the fibular and astragaloid 
lateral ligament. Its belly passes straight forwards to the proximal end of the fifth 
metatarsal, where a few of its fibres are inserted; but its strong flat tendon, joined by 
that of the second muscle above described, goes along the outer side of the fourth 
metatarsal, and is finally inserted by a broad tendinous expansion of the head of the 
proximal phalanx of that digit. This tendon is moreover strengthened by that of the 
peroneus quartus and peroneeus tertius. 
The distinction between the peronwus brevis in the Sea-horse and Sea-lion is its 
manner of insertion. In the former the tendon spreads out and has somewhat double 
points of fixture into the root and outer side of the fifth metatarsal bone (woodcut 6, 
Pb); in the latter there is less expansion. 
In the Walrus we meet with a deviation from the type of Eared and Earless Seals in 
the decided development of a peroneus quartus muscle; and, moreover, it is probable 
that the homologue of a peronzus tertius is also present. Both of these muscles, it is 
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