402 ELEMENTARY ANATOMY. [LESS. 



do not form a definite interlacement like the brachial plexus 

 of man and the higher Vertebrates. 



29. The NERVES OF THE LEG of man result from the inter- 

 mixture, in what is called the lumbar and sacral plexuses, of 

 the ventral branches of the four lowest lumbar nerves and of 

 the four highest sacral nerves, five of these (the lowest lumbar 

 and four sacral) contributing to form the sacral plexus. 



Issuing from the lumbar plexus is the crural nerve, which 

 goes to the front of the thigh and to the pectineus and 

 sartorius muscles ; while from the sacral plexus issues the 

 largest nerve in man's body the great sciatic, which divides 

 into the anterior tibial, supplying the extensors of the foot, 

 and the posterior tibial, supplying its flexors and thus ap- 

 parently answering to both the median and ulnar nerves of 

 the arm. 



The crural nerve passes out in front of the pelvis, but the 

 sciatic passes out behind it, between the great trochanter and 

 the tuberosity of the ischium. Its flexor branch (the pos- 

 terior tibial) passes on behind the knee-joint and under the 

 internal malleolus, while its extensor branch runs between the 

 gastrocnemius and biceps muscles round the head of the 

 fibula, and descends in front of that bone. 



The lumbar plexus exists in almost all Vertebrates above 

 Fishes, but it may be formed of only two nerves, as in Meno- 

 poma and the Crocodile, and it may be more separated from 

 the last nerve of the sacral plexus than in man, as is the 

 case in Birds. 



The sacral plexus, similarly constant, may be formed by 

 but three nerves, as in Menopoma, or but two, as in Proteus; 

 or it may, on the contrary, as is the case in Birds, be formed 

 by the concurrence of as many as six spinal nerves. As many 

 as four or five of these issue from foramina of the sacrum pre- 

 axial to those sacral vertebrae which are furnished with ex- 

 panded transverse processes, or sacral ribs (see Less. 1 1. p. 56). 



In Lizards also the sacral plexus is mainly formed by 

 nerves issuing from vertebrae pre-axial to those which articu- 

 late with the iliac bones^two out of the three nerves which 

 combine to form the sciatic nerve being thus conditioned. If 

 therefore we take the nerves as our standard of comparison, 

 the so-called sacrum of Lizards does not exactly answer to 

 that of man and Mammals generally, but is relatively a 

 somewhat more post-axial structure. 



A general agreement is again shown with regard to the 

 nerves of the leg in all Vertebrates with developed hind 



