X SPINAL CORD AND NERVES 515 



two bands — one (posterior pillar) lying along the anterior edge of the 

 hippocampus, and the other (anterior pillar) passing backwards in the 

 side walls of the third ventricle. Connecting the two optic thalami 

 arc three tranverse bands of nerve fibres, known respectively as the 

 anterior (a. co), middle {in, co) and posterior (p. co) commissures: the 

 middle commissure, which is much the largest, is not represented in the 

 lower Vertebrata. 



The spinal cord (Fig. 125, sp. c) is similar in structure 

 to that already described in other Vertebrates (pp. 155 and 

 443). It extends through the entire neural canal, ends in a 

 Jilum terminak, and is swollen opposite the fore- and hind- 

 limbs, where the nerves arise which form the limb-plexuses 

 (pp. 161 and 162). 



The dorsal and ventral roots of the spinal ncrvCS lie in the same 

 transverse plane, as in the frog (p. 163), but are relatively shorter than 

 in that animal ; and after uniting to form the nerve-trunks, pass directly 

 outwards through the intervertebral foramina. The brachial plexus is 

 formed from the four posterior cervical and the first thoracic nerve, and 

 gives off a number of nerves to the shoulder and fore-limb. The sciatic 

 or lumbosacral plexus is constituted by the two or three hindermosf 

 lumbar and the first two or three sacral nerves, and gives ofif branches to 

 the pelvic region and hind-limb, the chief of which are a femoral and 

 Peroneal going to the dorsal extensor muscles, and a large sciatic and an 

 obturator (which passes through the obturator foramen, p, 486) supplying 

 the ventral flexor muscles. Arising from the fourth cervical spinal 

 nerve of either side is a phrenic nerve (Fig. 12^, phr. n), which passes 

 backwards, between the heart and lungs, to supply the muscles of the 

 diaphragm ; and a large auricular nerve, arising from the third cervical 

 nerve, supplies the external ear. 



In addition to the ten cerebral nerves enumerated in the 

 frog (p. 163) and dogfish (p. 445), two others — the spinal 

 accessory and the hypoglossal (represented by the first spinal 

 nerve in the frog, p. 160) emerge from the skull and are 

 counted respectively as the eleventh and twelfth cerebral 

 nerves. The former arises from the side of the spinal cord 



L L 2 



