488 PISCES— FISHES. 



The brain.— The brain (see p. 438) has the following 

 parts : — 



1. The fused cerebral hemispheres or prosencephalon, with a 



nervous roof, and without ventricles. 



2. The thalamencephalon or region of the optic thalami, with 



a thread-like pineal body above, infundibulum and pituitary 

 body below, thinly roofed third ventricle within. 



3. The mesencephalon or mid-brain, with the optic lobes above, 



the crura cerebri below, the iter passing between. 



4. The cerebellum, with an anterior and a posterior lobe, both 



marked by ridges and grooves. 



5. The medulla oblongata, with thin vascular roof, with dorso- 



lateral extensions called "restiform bodies." 

 The region beneath the thalamencephalon bears — (a) two ovoid inferior 

 lobes ; (i) the infundibulum, which carries the pituitary body ; and (c) a 

 thin-walled three-lobed saccus vasculosus, situated between the pituitary 

 body and the inferior lobes. 



Cranial nerves. 1 — Owing to the flat form of the skate 

 and its frequently large size, the dissection of the cranial 

 nerves is perhaps easier than in any other Vertebrate. 

 Expecting practical verification, we shall describe their 

 distribution in some detail, following in regard to certain 

 points the investigations of Professor Cossar Ewart. 



I. The olfactory, rising from the olfactory lobes of the 

 cerebral hemispheres, extend to the nostrils, and 

 there expand in olfactory bulbs, which give off 

 small nerves to the nostrils. 

 II. The optic, leaving the region of the optic thalami, 

 cross in an optic chiasma, and extend to the 

 retina of the eye. 



III. The oculomotor or ciliary, arising from the crura 



cerebri, near the mid-ventral line, supply four of 

 the six muscles of the eye. There is a ciliary 

 ganglion in connection with III., and also with 

 the ganglion of the ophthalmicus profundus. 



IV. The pathetic or trochlear are small nerves emerging 



dorsally from between the mid- and hind- 

 brain, and supplying the superior oblique 

 muscles of the eye. It is possible that they 

 really belong to V. 

 V. The trigeminal, or nerve of the "mouth-cleft," 



1 I have to acknowledge indebtedness to Dr. Beard for his kindness 

 in helping me to state the distribution of these nerves. 



