v. i 4 -i5 BRAIN OF ELASMOBRANCHS 167 



The sympathetic ganglia are closely associated with these suprarenal 

 bodies, as would be expected from their common origin from cells 

 of the neural crest. The segmental series continues along the whole 

 length of the abdomen, the more posterior members being embedded 

 in the kidney tissue (Fig. 105). These posterior suprarenal bodies are 

 larger in the male than in the female, but only the central part of the 

 male glands shows the reaction with chrome salts that indicates the 

 presence of adrenaline. The peripheral portion of each gland appears 

 to consist of non-functioning cells, possibly a reserve used only during 

 reproduction (see p. 164). 



The part of the adrenal corresponding to the cortex of mammals is 

 represented in elasmobranchs by the interrenal bodies, lying medially 

 in some species, paired in others, in the kidney region (Figs. 105 and 

 106). The cells of these organs resemble cortical adrenal cells. Since 

 they are not in contact with the suprarenals at any point, it would seem 

 that the association of the two parts is not necessary for their function- 

 ing, at least in these animals. Removal of the interrenal is always fatal. 

 The gland is stimulated by 'stress' or by mammalian ACTH. Extracts 

 of it prolong the life of adrenalectomized rats. There is evidence that 

 it influences carbohydrate metabolism and activity of the gonads but 

 not electrolyte balance. 



The islets of Langerhans contain two cell types as in mammals. 

 The pineal body is small and without any trace of eye-like structure. 



The gonads contain endocrine organs, producing steroid hormones. 

 These are formed by interstitial cells in the testes. Oestrogens probably 

 come from the outer (theca) cells of the follicles that surround the 

 eggs. The inner (granulosa) cells of the capsule assist in yolk produc- 

 tion but may also produce progesterone and in viviparous species 

 they develop into a distinct corpus luteum after ovulation. 



15. Nervous system 



The brain is large and well developed in elasmobranchs, having a 

 structure characteristically different from that of both the cyclostomes 

 and bony fishes (Fig. 100). The forebrain is large and has cerebral 

 hemispheres thickened both in floor and roof, whereas in teleosts the 

 roof is thin. The hemispheres are wide relative to their length and the 

 end of the unpaired portion of the forebrain between the hemispheres, 

 the lamina terminalis, is also much thickened. Attached to the ends 

 of the cerebral hemispheres are large olfactory bulbs and there are 

 also large nasal sacs. Evidently the olfactory sense is well developed 

 in these animals and they depend greatly on it for hunting. 



