198 THE VOYAGE OF H.M.S. CHALLENGER. 



In Arctocephalus gazella it is beneath the peroneus quinti digiti, and arises from the dorsum of 

 the head of the fibida below the peroneus longus, from the whole extent of its outer border, and 

 from the posterior three-fourths of its dorsal surface. It has the same course as the peroneus 

 quinti digiti, and is inserted into the dorsal proximal surface of the 5th metatarsal bone, but 

 before gaining the bone it broadens considerably. In Otaria it is very much the same as in Arcto- 

 cephalus. In Trichechus the tendon does not expand so much as in Otaria, In the Phoeinae it is 

 supplied by the external popliteal nerve. 



In all the specimens the peroneus tertius and the peroneus quartus are absent. 



In Trichechus are found the peroneus tertius and the peroneus quart i digiti, which latter is 

 diminutive. The peroneus longus is an extensor of the ankle, the peroneus brevis and the peroneus 

 quinti digiti are flexors and abductors of the foot, and the brevis and quinti digiti expand the toes. 



The Inner Tibiofibular Eegion consists of a superficial and a deep group of muscles. 



The Superficial Group in Phoca vitulina, Phoca hispida, Phoca barbata, and Macrorhinus 

 leonins is formed by the gastrocnemius and plantaris. In Arctocephalus gazdla, besides the other 

 muscles, there is the soleus. 



The Gastrocnemius in the Phocinae is a two-headed muscle, and the inner head is more than 

 double the size of the outer. The inner head arises from the back of the femur above the internal 

 condyle, reaching up the shaft to the junction of the internal border with the supracondyloid ridge, 

 from the internal surface of the same condyle above the fossa for the internal lateral ligament, from 

 the internal lateral hgament extending to the junction of the anterior third with the posterior two- 

 thirds of the tibia, from the anterior third of the ventral border of the shaft ventral to the lateral 

 hgament, and from the capsular ligament of the knee-joint. The outer head arises from the outer 

 surface of the external condyle in common with the plantaris muscle, slightly from the outer half 

 of the surface of the femur above the same condyle, and by a few fibres from the back of the head 

 of the fibula. The two heads unite opposite the junction of the middle two-thirds with the 

 posterior third of the tibia, and form a tendon which widens near the os calcis, and is inserted into 

 the anterior aspect of the tuberosity of the os calcis. 



In Macrorhinus leoninus the inner head arises as in Phoca vitulina, but covers more of the back 

 of the femur, also from the front surface of the internal condyle up to the patellar facet of the 

 femur. The outer head does not arise from the femur, but from the inner dorsal surface of the 

 head of the fibula. The fibres of the inner head join those of the outer head at the anterior third 

 of the tibia, and form a strong tendon, which is inserted as in Phoca vitulina. 



In Arctocephahis gazella it is a single-headed muscle, and arises from the inner surface of the 

 internal condyle of the femur below the fossa for the internal lateral hgament, from the internal 

 lateral hgament, from the internal border of the tibia in its upper third, and from the capsule of 

 the knee-joint. It crosses the leg from the dorsal to the ventral side; one inch from the os calcis 

 it forms a tendon, which widens and is inserted into the os calcis to the outer side of the groove for 

 the plantaris tendon. 



Humphry and Lucae give no connection with the fibula ; the bony attachments are the same in 

 Otaria and Trichechus. In the Phocinae and Macrorhinus it will powerfully extend the foot when 

 swimming ; in Arctoccplialus it also extends the foot in the water, and raises the heel in walking. 

 In the Phocinaa it is supplied by the great sciatic nerve. 



