IX 



NERVOUS SYSTEM 



NERVOUS SYSTEM OF HEPTANCHUS MACULATUS 



Central Nervous System 

 BRAIN 



The brain of Heptanchus maculatiis may be described as made up of five 

 divisions, as is common for the Elasmobranchs. These divisions, beginning 

 anteriorly are : the telencephalon, the diencephalon. the mesencephalon, the 

 metencephalon, and the mj^elencephalon. 



The telencephalon ( //., fig. 200a ) , if seen 

 from the dorsal side, appears as a bilobed 

 mass which is continued forward by long 

 olfactory tracts (ol.t.). Between the tracts 

 and projecting slightly anteriorly is the 

 median olfactory nucleus (m.n.), better 

 seen in ventral view (fig. 200b). Dorsally 

 the telencephalon is raised up into the so- 

 called pallial eminences. At the angle be- 

 tween the median olfactory nucleus and 

 the pallial eminence is the recessus neuro- 

 porieus, at the sides of which arises the 

 terminal nerve (tn.). The telencephalon 

 is continued posteriorly by the diencepha- 

 lon (fZf, fig. 200b). 



The diencephalon is provided with a 

 thin roof through which the pineal stalk 

 passes as a slender thread upward and 

 forward to the cartilaginous roof. This 

 segment of the brain continues posteriorly 

 as a gradually narrowing mass back to the 

 place where the optic nerves (//) form 

 the optic chiasma. From the posterior and 

 ventral part of this division arises the in- 

 fundibulum, at the sides of which are the 

 inferior lobes (i.l.) and the vascular sacs 

 (v.s.) of the brain. In the middle line and ventral to the vascular sacs are the 

 lobes of the pituitary (see fig. 215) . 



The mesencephalon is well developed (ins., fig. 200b) . Dorsally it consists of 

 two hollow optic lobes (op.l., fig. 200a) or corpora bigemina. These in their 

 posterior part are overlapped by the cerebellum (cb.) and ventrally by the 

 infundibnlum and its associated structures. Through the posterior roof of 



md. V. 



Fig. 200b. Brain and cranial nerves, 

 Heptanchus maculafus, ventral view. 

 For explanation see fig. 200a. 



[221] 



