SECRETION. 257 



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 kiduey 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. 389.) 



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 duet 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 that sets 

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

 ples which so frequently occur upon the skin from occlusion of the opening of 

 the ducts. The exact composition of the secretion is not known. It contains 

 fats and soaps, some cholesterin, albuminous material, pari of which is a 

 nucleo-albumin often described as a casein, remnants of epithelial cells. 

 and inorganic salts. The cholesterin occurs in combination with a fatty acid 

 and is found in especially large quantities in sheep's wool, from which it is 

 extracted and used commercially under the name of lanolin. 'The sebaceous 

 secretion from different places, or in different animals, is probably somewhal 

 variable in composition as well as in quantity. The secretion of the prepuce 

 is known as the smegma prcepvMi; that of the external auditory meatus, 

 mixed with the secretion of the neighboring sweat-glands or ceruminous glands, 

 forms the well-known ear-wax or cerumen. The secretion in this place con- 

 tains a reddish pigment of a bitterish-sweet taste, the composition of which has 

 not been investigated. Upon the skin of the newly-born the sel»a< us ma- 



Vol. I.— 17 



