112 PSYCHOBIOLOGY 



walls of the pancreatic duct contain smooth muscle fibers. Sometimes there 

 is a secondary pancreatic duct. 



Both the liver and the pancreas are supplied with nerve fibers from the 

 solar plexus. Most of the fibers are sympathetic, but apparently there are 

 some derived from the vagus nerve. Some of these fibers run to the mus- 

 cular coats of the blood vessels and the ducts of the glands, and some run 

 to the gland cells themselves. The details of nervous control of the liver 

 and pancreas are obscure. The liver secretes continuously, but more 

 copiously during digestion ; the principal stimulus to the activity of both 

 liver and pancreas has been shown to be the secretin manufactured by the 

 small intestine, carried in the blood to the glands and acting directly on 

 the gland cells. The same hormone also causes contraction of the gall 

 bladder, emptying its contents through the common bile duct. 



GLANDS OF THE SKIN. 



The chief skin glands are the sweat glands {sudoriparous or sudo- 

 riferous glands) and the sebaceous glands [Fig. 57]. The former are 

 coiled simple tubes, the latter are alveolar. 



The sebaceous glands are usually associated with hair, the ducts of 

 one to four glands opening into the superficial part of the hair follicle. 

 On some part of the body (the lips, for example), the glands open on the 

 surface independently of the hairs. The active cells in these glands secrete 

 by forming sebum within themselves and then liberating it by breaking 

 down ; new cells from the deeper layer replacing the dissolved ones. There 

 are no nerve fibers supplied to these glands and no muscular fibers in the 

 glands themselves. The contraction of the arrector pili muscle attached 

 to a hair follicle (raising the hair follicle and thus producing the condition 

 known as "goose flesh") compresses the sebaceous gland and squeezes 

 out the sebum. These muscles are controlled by fibers from the sympa- 

 thetic division of the nervous system. 



The sweat glands are under the direct control of the sympathetic nerve 

 fibers which terminate both on the secreting cells lining the tube and the 

 smooth muscle fibers which lie next to these cells. These secretory nerve 

 fibers are derived from spinal nerve roots from the second dorsal to the 

 third or fourth lumbar. The normal stimulus for the sweat-reflex is 

 heat applied to the surface of the body through external sources, or an 

 increase in the temperature of the blood within the body. Concerning 

 the mechanism of the excitation of the afferent currents which control 

 the efferent current to the sweat glands, there is meager information. 



The above list does not include all the duct glands, nor even all the im- 

 portant ones, but is sufficiently extended to give an elementary idea of the 

 general facts of duct-gland structure and function. 



