Xii PHYLUM CHORDATA 381 



surface is marked with a few fine grooves. The medulla 

 oblongata {NfJ), broad in front, narrows posteriorly to pass 

 into the spinal cord. The fourth ventricle {F. rho) is a 

 shallow space on the dorsal aspect of the medulla oblongata 

 covered only by a thin vascular membrane, the choroid 

 plexus ; it is wide in front and gradually narrows posteriorly. 

 At the sides of the anterior part of the fourth ventricle are a 

 pair of folded ear-shaped lobes, the corpora restiformia. 



The fourth ventricle is continuous behind with the cen- 

 tral canal of the spinal cord. In front it is continuous with 

 a narrow passage, the iter {iter), which opens anteriorly into 

 a wider space, the diaccele or third ventricle (dia) occupying 

 the interior of the diencephalon. From this opens in front 

 a median prosocxle, which gives off a pair of paracoeles 

 {para) extending into two lateral portions of the prosen- 

 cephalon. 



A series of nerves arise in pairs from the brain and spinal 

 cord. From the spinal cord the nerves arise segmentally, 

 one pair corresponding to each myomere, and pass through 

 apertures in the neural arches of the vertebrae. Each arises 

 by two roots, a dorsal and a ventral. The dorsal root is 

 dilated into a ganglion, and contains only sensory fibres ; 

 the ventral root is non-ganglionated, and is motor. A 

 longitudinal ganglionated sympathetic nerve, extending along 

 the dorsal region of the ccelome, is connected with the 

 spinal nerves, and sends branches to the viscera, blood- 

 vessels, etc. 



From the brain arise ten pairs of nerves, some of which 

 are sensory, others motor, others mixed. Three are the 

 nerves of the principal sense organs : the first, or olfactory, 

 supplying the organ of smell (Fig. 228, olf. s) ; the second, 

 or optic, the retina of the eye, and the eighth, or auditory, 

 the organ of hearing. The third, or oculomotor, the fourth, 



