SECRETION. 



441 



ducts to be discharged on the surface of the mucous membrane or skin, as the 

 case may be. 



Influence of the Nerve System. The activity of every gland is con- 

 trolled by nerve-centers situated in the central nerve system. These centers 

 may be excited to activity either by impressions made on the peripheral 

 terminations of afferent nerves or by emotional states, or, possibly, by changes 

 in the composition of the blood itself. As a rule, all normal secretion is a 

 reflex act involving the usual mechanism: viz., a receptive surface (skin, 

 mucous membrane, or sense-organ), an afferent nerve, an emissive cell from 

 which emerges an efferent nerve to be distributed to a responsive organ, the 

 gland epithelium. 



For the production of the secretion by the epithelial cell it is believed 

 by some experimenters that two physiologically distinct, efferent nerve- 

 fibers are involved one stimulating the production of the organic constituents 

 (trophic nerves), the other stimulating the secretion of water and inorganic 

 salts (secretor nerves). The evidence for the influence of the nerve system 

 on secretion and the mode of connection of the nerve-fibers with the gland- 

 cells have been alluded to (page 94) and will again be in subsequent chapters. 



The reticular and vascular glands, though not possessing any common 

 histologic features, are grouped together merely for convenience, and will 

 be considered in a separate chapter in connection with the problems of 

 internal secretion. 



MAMMARY GLANDS. 



The mammary glands, which secrete the milk, are two more or less 

 hemispheric organs situated in the human female on the anterior surface of 



the thorax. Though rudimentary in 

 childhood, they gradually increase in 

 size as puberty approaches. The gland 

 presents at its convexity a small conical 

 eminence termed the mammilla or nipple, 

 surrounded by a circular area of pig- 

 mented skin, the areola. The gland 

 proper is covered by a layer of adipose 



FIG. 204. MAMMARY GLAND. i. 

 Lactiferous ducts. 2. Lobuli of the 

 mammary gland. 



FIG. 205. ACINI OF THE MAMMARY 

 GLAND OF A SHEEP DURING LACTATION. 

 a. Membrana propria. b. Secretory 

 epithelium. 



tissue anteriorly and is attached posteriorly to the pectoral muscles by a net- 

 work of fibrous tissue. 



