■REPORT ON THE SPHENISCID^. 165 



Vanations in Crural Plexus. — In none of the birds which I have dissected, with the 

 single exception of Eudyptes ch^socome from Tristan, does the first lumbo-sacral nerve 

 contribute in any way to the formation of the crural plexus. 



In Spheniscus minor the crural plexus is formed by the second and third lumbo- 

 sacral nerves alone. The fourth nerve, which in all the other species forms part of the 

 crural, in this species joins the sciatic plexus. 



In Aptenodytes the branch to the abdominal muscles, which is derived from the 

 second lumbo-sacral nerve, and is usually distributed separately, unites with the first 

 lumbo-sacral nerve and forms a portion of the latter. 



The Sciatic Plexus. 



The anterior division of the fifth lumbo-sacral nerve (fig. 2), so soon as it passes 

 from the intervertebral foramen, receives a large communicating branch from the fourth, 

 and then unites with the sixth, seventh, and eighth lumbo-sacral nerves to form the 

 sciatic plexus. 



Thus formed the sciatic plexus gives origin to the sciatic nerve. 



TJie Sciatic Nerve. 



The sciatic nerve (15) is formed by the union of a portion of the fourth with the 

 whole of the fifth, sixth, seventh, and eighth lumbar nerves. It escapes from the pelvic 

 cavity through the sciatic foramen. Having reached the thigh, the nerve divides into 

 two parts — a superficial and a deep. The superficial portion lies between the biceps and 

 the adductor longus, the deep between the latter and the adductor magnus muscles. The 

 deep portion at once breaks up into branches for the supply of the semi-membranosus 

 (16) and adductor longus (17) muscles. 



(A) Tlie superjicial portion of the sciatic nerve, after giving ofi" a number of twigs for 

 the supply of the biceps femoris (18), breaks up into the following branches : — 



(a) A cutaneous branch (19) which extends downwards along the back of the calf of 

 the leg, as low as the sole of the foot. It gives ofi" branches to the skin as it descends. 



(6) A branch of supply to the outer head of the gastrocnemius (20). 



(c) A branch to the superficial portion of the Jlexor po for atus digitorum and plantaris 

 (21). 



(d) A long slender branch (22) which extends downwards along the whole length of the 

 calf, lying between the flexor perforatus and the inner head of the gastrocnemius. 

 Immediately above the ankle joint this nerve passes forwards beneath the tendon of 

 insertion common to the anterior and inner heads of the gastrocnemius, and having 

 gained the front of the metatarsus, descends as far as the sole of the foot, to be distri- 

 buted to the skin in that region. About the middle in length of the tibial segment of 



