532 THE RABBIT CHAP. 



side with 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 are three 

 transverse bands of nerve-fibres, known respectively as the 

 anterior (a. co), middle (m. co), and posterior (p. co) commis- 

 sures : the middle commissure, which is much the largest, 

 is not represented in the lower Vertebrata. 



The spinal cord (Fig. 135, sp. c) is similar in structure 

 to that already described in other Vertebrates (pp. 155 

 and 459). It extends through the entire neural canal, 

 ends in a filum terminate, 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 nerves lie in the 



same transverse plane, as in the frog (p. 163), but are rela- 

 tively shorter than in that animal ; after uniting to form the 

 nerve-trunks, they pass directly outwards through the inter- 

 vertebral foramina. The brachial plexus is formed from the 

 four posterior cervical and the first thoracic nerves, 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 hindermost lumbar and the first two or three sacral 

 nerves, and gives off branches to the pelvic region and hind- 

 limb, a femoral and a peroneal going to the extensor muscles, 

 and a large sciatic and an obturator (which passes through 

 the obturator foramen, p. 503) supplying the flexor muscles. 

 Arising from the fourth cervical spinal nerve of either side 

 is a phrenic nerve (Fig. 135, 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. 461), two others the 

 spinal accessory and the hypoglossal (represented in the 

 frog by fibres in connection with the vagus and by the 

 first spinal nerve respectively, p. 160) emerge from the 

 skull and are therefore counted as the eleventh and 

 twelfth cerebral nerves. The former arises from the side 

 of the spinal cord and bulb by numerous fibres, the 



