CHAPTER 3 

 KINETIC HORMONES 



I. CONTROL OF MUSCLES AND 

 PIGMENTARY EFFECTORS 



The term "kinetic" (§ L51) brings together a large group of 

 hormones, which act upon certain effectors in the organism in 

 ways which often resemble the effects of nerve stimulation. 

 Kinetic hormones acting upon muscles and pigmentary effectors 

 are considered in this chapter, and those causing the secretion of 

 glands, both exocrine and endocrine, in the next (§§ 4.1 and 4.2). 



The similarity in action between these kinetic hormones and 

 some nerves is not entirely accidental, since many of the kinetic 

 hormones are neurosecretions from modified nerve cells (§2.111), 

 and some are chemically akin to the acetylcholine or noradrenaline 

 secreted by cholinergic and adrenergic nerves respectively. An 

 important difference lies in their means of distribution; the hor- 

 mone reaches the effector through the blood circulation and is 

 therefore widespread in its effect, whereas the nerve cell releases 

 its chemical in contact with a single effector only. 



There are, however, other kinetic hormones that are not derived 

 directly from nervous tissue. Some of these, like those from the 

 corpus allatum of insects or the adenohypophysis of vertebrates, are 

 likewise derived from the ectoderm. A few others, such as secretin, 

 appear to be derived from neither nerve cells nor ectoderm, but 

 from the endoderm (§ 2.22). The kinetic hormones therefore form 

 a wider group than either ''neurohormones" (Welsh, 1955) or the 

 secretions of "neurohaemal organs" (Carlisle and Knowles, 1953) ; 

 yet they show quite sufficient functional similarity among them- 

 selves to warrant their inclusion in one group. 



The means of stimulating their secretion may be mechanical, as 



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