532 THE RABBIT CHAP. 



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 tranverse bands of nerve fibres, known respectively as the 

 anterior (a. c0), middle (m. 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. 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 relatively shorter than 

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

 outwards through the intervertebral 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 sciatu 

 or lumbo-sacral 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, the chief of which are a fentoral 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 



