66 



KINETIC HORMONES — I 



down into the teats, achieving what is known to farmers as "let- 

 down" of the milk. Figure 3-7 shows that injection of oxytocin 

 into an anaesthetized bitch, Canis, stimulates the contraction of 

 these cells, and increases the amount of milk available to the 

 puppies. In the normal state, the act of suckling (or milking) stimu- 

 lates the brain to induce secretion of oxytocin for this purpose. 



Fig. 3-6. Diagram of a generalized exocrine gland to show the 

 location of factors which may influence the flow of secretion. 

 Duct activity influenced by: 1, smooth muscle sphincters; 2, longi- 

 tudinal smooth muscle shortening ducts or producing peristalsis ; 

 3, myoepithelium; 4, reservoirs in large ducts, or cisterns; 5, 

 vasodilatation pressing on ducts or reservoirs; 6, vasoconstriction 

 shortening inter-lobular vessels and squeezing adjacent ducts; 

 7, secretion from duct epithelium. 



Lobule activity influenced by: 8, smooth muscle bundles in inter- 

 lobular septa squeezing lobules as a whole; 9, smooth muscle 

 interspersed between alveoli; 10, vasodilatation or vasoconstriction 

 affecting alveoli mechanically; 11, myoepithelium; 12, elastic fibre 

 recoil in stroma, when pressure in distended alveoli is released; 



13, nervous stimuli to secretory epithelium and smooth muscle; 



14, hormonal stimuli to epithelium. In mammary glands the 

 myoepithelial cells, 11, round the lobules, contract in response to 

 OXYTOCIN. The secretory epithelium, 14, is stimulated by prolactin 



(§4.13). (From Richardson, 1949). 



