SECRETION. 197 



anterior roots of the eleventh, twelfth, and thirteenth spinal nerves. Under 

 normal conditions these fibres are probably thrown into action by reflex stimula- 

 tion and lead to an increased functional activity. It will be seen, therefore 

 that the kidneys possess a local nervous mechanism through which their 

 secretory activity may be increased or diminished by corresponding alterations 

 in the blood-supply. So far as is known, this is the only way in which the 

 secretion in the kidneys can be directly affected by the central nervous system. 

 It should be borne in mind, also, that the blood-flow through the kidneys, 

 and therefore their secretory activity, may be affected by conditions influ- 

 encing general arterial pressure. Conditions such as asphyxia, strychnin- 

 poisoning, or painful stimulation of sensory nerves, which cause a great 

 rise of blood-pressure, influence the kidney in the same way, and tend, 

 therefore, to diminish the flow of blood through it ; while conditions which 

 lower general arterial pressure, such as general vascular dilatation of the skin 

 vessels, may also depress the secretory action of the kidney by diminishing 

 the amount of blood flowing through it. 



In what way any given change in the vascular conditions of the body will 

 influence the secretion of the kidney depends upon a number of factors, and 

 their relations to one another ; but any change which will increase the differ- 

 ence in pressure between the blood in the renal artery and the renal vein will 

 tend to augment the flow of blood unless it is antagonized by a simultaneous 

 constriction in the small arteries of the kidney itself. On the contrary, any 

 vascular dilatation of the vessels in the kidney will tend to increase the blood- 

 flow through it unless there is at the same time such a general fall of blood- 

 pressure as is sufficient to lower the pressure in the renal artery and reduce the 

 driving force of the blood to an extent that more than counteracts the favora- 

 ble influence of diminished resistance in the small arteries. 



Movements of the Ureter and the Bladder. (See Micturition, p. 327.) 



E. CUTANEOUS GLANDS ; INTERNAL SECRETIONS. 



The sebaceous glands, sweat-glands, and mammary glands are all true epider- 

 mal structures, and may therefore be conveniently treated together. 



Sebaceous Secretion. The sebaceous glands are simple or compound 

 alveolar glands found over the cutaneous surface usually in association with the 

 hairs, although in some cases they occur separately, as, for instance, on the pre- 

 puce and glans penis, and on the lips. When they occur with the hairs the 

 short duct opens into the hair-follicle, so that the secretion is passed out upon 

 the hair near the point where it projects from the skin. The alveoli are filled 

 with cuboidal or polygonal epithelial cells, which are arranged in several lay- 

 ers. Those nearest the lumen of the gland are filled with fatty material. 

 These cells are supposed to be cast off bodily, their detritus going to form the 

 secretion. New cells are formed from the layer nearest the basement mem- 

 brane, and thus the glands continue to produce a slow but continuous secretion. 

 The sebaceous secretion, or sebum, is an oily semi-liquid material which sets 

 upon exposure to the air to a cheesy mass, as is seen in the comedones or pirn- 



