DEVELOPMENT 635 



basement membrane, and some come to lie in the lumen of 

 the tubules and the cavities of the alveoli. These latter 

 underffo fattv degeneration and are shed with the first 

 milk — colostrum. It is the cells left on the basement of 

 the membrane of the alveoli that elaborate the constituents 

 of the milk. 



(2) Regulation of Activity — (a) Chemical Stimulation. — This 

 subject is dealt with on page 609. (6) Nerve Stimulation. — 

 The extent to which the secretion of milk is intluenced by 

 the nervous system has not been determined with certainty. 

 After secretion of all the nerves passing to the gland, if the 

 animal be lactating, secretion continues, though the amount 

 may be diminished ; if the animal be pregnant, glandular 

 development proceeds, and at parturition normal secretion 

 of milk occurs. On the other hand, pain or excitement 

 reduces the quantity of milk. AVhatever influence the 

 nervous system does exert is probably produced through 

 vasomotor nerves and intrinsic nerves of the gland. On the 

 whole, it seems certain that control through the nerves is 

 subsidiary to chemical control through the blood. 



The flow of the milk can be influenced by the central 

 nervous system. The walls of the ducts contain muscle 

 fibres which can act by constricting the lumen and stopping 

 the flow. In this way the cow is able to "hold up" its 

 milk, as often occurs when milking is attempted by a person 

 to whom the animal is unaccustomed, or when a cow that 

 has been sucked by its calf is milked by hand. (c) 

 Mechanical Stimulation. — The distension of the ducts with 

 milk inhibits further secretion. The periodical emptying of 

 the udder, therefore, by sucking or milking is necessary to 

 maintain functional activity. The influence of the sucking 

 is not entirely due to the relief of the distension. The 

 mechanical stimulation probably induces secretion through a 

 nervous reflex, as bv this means a flow of milk mav be 

 produced in a virgin animal (p. 609). 



3. Composition of Milk — (1) Adaptation for Needs of Young 

 Animal- — Milk is produced to supply material and energy 

 to a rapidly growing animal. The materials present, there- 

 fore, are in proportion to the requirements for growth. It 



