NERVES OE RErTILES. 3)5 



most of Avhich pass through, and terminate in the vascular beds of 

 the horny scutes. 



The seventh and eiglith dorsal nerves, and the three consecutive 

 pairs, contribute to the formation of the crural plexus. The 

 sciatic nerve is formed by the last dorsal and the first two sacral 

 nerves. In the Crocodiles and Lizards the sciatic is formed by 

 but two spinal nerves : in the Frogs and Toads by three, 

 figs. 207, 208. 



The crural plexus in the Tortoise sends filaments to the trans- 

 versalis, fig. 150, 4i, and obliquus abdominis, fig. 151, 40 ; to some 

 of the pelvic muscles and the glutasi ; it is then continued into the 

 limb as the 'crural nerve.' The obturator nerve is a direct branch 

 of the last dorsal. The sciatic nerve gives a filament to the 

 second gluticus and to the obturatorius, and continues, as a 

 large trunk, to behind the knee-joint, where it divides into 

 the tibial and peroneal nerves. The tibial subdivides into a 

 popliteal branch, supplying the muscles at the back of the leg 

 and the sole or plantar side of the foot, and into an external 

 branch to the external muscles and integument. In the Turtle 

 (Chelone),^ one division of the sciatic gives branches to the 

 muscles of the thigh, and is continued to the plantar surface 

 of the foot, dividing into digital nerves, terminating on 

 the skin ; the other division, after giving off some muscular 

 branches, passes to the skin on the dorsal surface of the fin. 



In Lizards the crural nerve is formed by the two lumbar nerves, 

 and is distributed to the muscles on the fore-part of the thigh ; 

 the sciatic nerve is formed by the last lumbar, the two sacral 

 nerves, is continued along the inner side of the thigh, supjjlying 

 the muscles as far as the digits, and branching to accompany them. 



In the Frog a pearly vesicle, with calcareous molecules, covers 

 each spinal nerve where it comes out of the spinal canal. Of the 

 four pairs of nerves which proceed from the termination of the 

 short myelon, three constitute on each side the ' sciatic plexus,' 

 which unites into a single large nerve opposite the acetabulum. 

 The sciatic nerve enters the muscles at the back-part of the thigh, 

 supplies them, and divides near the knee-joint into an external 

 and internal branch, distributed to the muscles, digits, and skin of 

 the hind-leg and foot. 



The tenacity of vital force in Hamatocrya, and the seemingly 

 peculiar susceptibility to the voltaic current in the Frog, have 

 made that animal the usual subject of the experiments exemplifying 



' Liv. pi. xvi. 



