§ 2.114 SECRETORY CELLS FROM THE NERVOUS SYSTEM 37 



is the more abundant in the paraventricular nucleus (Van Dyke, 

 Adamsons and Engel, 1957). 



These hormones are concerned with counteracting thirst and 

 the desiccation that is the main risk accompanying the migration 

 from water to land. The antidiuretic hormone facilitates reabsorp- 

 tion of water from the urine (§ 5.322) and oxytocin increases the 

 excretion of Na+ and CI" (§ 5.312). It is therefore understandable 

 that the neural lobe, where these hormones can be quickly released 

 into the blood, should be best developed in terrestrial animals 

 (Harris, 1955). The correlation has been confirmed by the observa- 

 tion that natural thirst, or an equivalent state caused by injecting 

 rats with saline, is followed by depletion of the secretory granules 

 within a few minutes, to be slowly replaced in a day or so after 

 giving the animals water to drink. Depletion of secretory granules 

 in fish has been observed in response to immersion in hypertonic sea 

 water, which would have the same effect as desiccation (Arvy, 1957). 



Gland cells of the suprarenal tissue and adrenal medulla 



The peripheral neurons of the sympathetic nervous system 

 all secrete adrenaline, or noradrenaline, at their motor nerve 

 endings. In most vertebrates some of these neurons become 

 modified to secrete relatively enormous amounts of either or both 

 of these substances ; at the same time the cells lose all histological 

 resemblance to ganglion cells (Fig. 2-1/). Their origin and func- 

 tion is nevertheless the same as that of neurosecretory cells ; but 

 opinion is divided as to whether they should be regarded as such 

 (Welsh, 1955). They are often referred to as "chromaffin" cells, 

 because the contained adrenaline gives a characteristic olive-brown 

 colour with any chromic salts used either as fixative or stain. 

 Staining shows that the adrenaline is secreted by the cytoplasm 

 as a mass of very fine granules. 



In fish, where these cells form the ''suprarenal" tissue, they 



in most tetrapods. Heavy arrows indicate the presence of portal 

 veins between primary venous plexus in median eminence and 

 secondary plexus in pars distalis (cf. Fig. 2-12) in fully terrestrial 

 forms. O.C, optic chiasma; S.V., saccus vasculosus; V.L., ventral 

 lobe (from Green, 1951). 



