REPORT ON THE SPHENISCID^. 151 



exceptional arrangement of these arteries obtains also, as I am informed by Mr. Forbes, 

 in certain other genera, but in none that can be supposed to be in any way allied to the 

 Penguins. 



The sciatic artery gives oflF two branches — the posterior tibial and the anterior tibial. 



(a) The Posterior Tibial Artery extends from the back of the knee joint downwards 

 to below the ankle. In this course the artery is quite superficially placed, being covered 

 only by the skin, and resting against the inner head of the gastrocnemius. It terminates 

 as a small vessel which extends as far down as the middle in length of the metatarsal 

 bone. As the posterior tibial artery passes downwards it gives otf numerous small 

 branches to the outer and inner heads of the gastrocnemius muscle. 



(b) The Anterior Tibial Artery extends from the termination of the sciatic artery 

 behind the knee joint forwards between the inner and outer heads of the gastrocnemius, 

 to reach the posterior surface of the tibia above the popliteus muscle. Here it comes into 

 contact with the posterior surface of the interosseous membrane, and passing downwards 

 under cover of the flexor perforans, as far as the middle in length of the tibia, pierces 

 that membrane and gains its anterior surface. Here the artery lies, along with the 

 anterior tibial nerve, between the tibialis anticus and the peroneus longus, and coursing 

 as far as the annular ligament above the ankle joint, terminates by dividing into the 

 external and internal digital arteries. 



The anterior tibial artery gives off the following branches. 



(a) The Articular Artery comes off from the anterior tibial at the upper border of the pop- 

 liteus. It passes forwards and upwards to supply the structures surrounding the knee joint. 



(b) Muscular Arteries. — These are three in number, and arise from tlie parent trunk 

 under cover of the popliteus muscle. They pass transversely forwards in the interval 

 between the tibia and fibula, and gaining the front of the leg, are distributed to the muscles 

 of that region, and more especially to the tibialis anticus and extensor digitorum. One of 

 these branches inosculates with the anterior tibial after the latter has pierced the interosseous 

 membrane. 



In one specimen of Aptenodytcs one of these muscular arteries was of large size, and 

 extended as far as the cleft between the two outer toes, where it took the place of the 

 external digital artery. In others the arrangement was similar to that described in Eiodyptes 

 chrysocome. 



(c) The Internal Diyital Artery, aibev separating from the anterior tibial, passes together 

 with the anterior tibial nerve and the tendon of the tibialis anticus beneath the anterior 

 annular ligament of the ankle, and having passed to the upper end of the metatarsus, divides 

 iuto two branches. Of these one, which may be named the perforating artery, passes from 

 front to back of the leg by means of a foramen between the middle and inner metatarsal 

 bones, while the other continues along the front of the metatarsus, as far as the base of the 

 toes, where it divides into two branches for the supply of the contiguous sides of the middle 

 and inner toes. 



(d) The External Digital Artery passes downwards in front of the metatarsus, lying to 

 the outer side of the anterior annular ligament, and opposite the head of that bone divides, 

 like the internal digital artery, into two branches. Of these one (the perforating) passes 



