180 HUMAN PHYSIOLOGY. 



from the blood ; it is so important that if elimination be prevented, 

 death occurs in a short time. 



Influence of the Nerve System. The secretion of sweat is regu- 

 lated by the nerve system. Here, as in the secreting glands, the 

 fluid is formed from material in the lymph-spaces surrounding the 

 gland. Two sets of nerves are concerned viz., vasomotor, regulating 

 the blood-supply ; and secretor, stimulating the activities of the gland 

 cells. Generally the two conditions, increased blood flow and in- 

 creased glandular action, coexist. At times profuse clammy perspira- 

 tion occurs, with diminished blood flow. 



The dominating sweat-center is located in the medulla, though 

 subordinate centers are present in the cord. The secretory fibers 

 reach the perspiratory glands of the head and face through the 

 cervical sympathetic ; of the arms, through the thoracic sympathetic, 

 ulnar, and radial nerves ; of the leg, through the abdominal sympa- 

 thetic and sciatic nerves. 



The sweat-center is excited to action by mental emotions, in- 

 creased temperature of blood circulating in the medulla and cord, in- 

 creased venosity of blood, many drugs, rise of external temperature, 

 exercise, etc. 



THE CENTRAL ORGANS OF THE NERVE 

 SYSTEM AND THEIR NERVES. 



The central organs of the nerve system are the encephalon and the 

 spinal cord, lodged within the cavity of the cranium and the cavity of 

 the spinal column respectively. The general shape of these two portions 

 of the nerve system correspond with that of the cavities in which 

 they are contained. The encephalon is broad and ovoid, the spinal 

 cord is narrow and elongated. 



The encephalon is subdivided by deep fissures into four distinct, 

 though closely related portions: viz., (i) the cerebrum, the large 

 ovoid mass occupying the entire upper part of the cranial cavity ; 

 (2) the cerebellum, the wedge-shaped portion placed beneath the 

 posterior part of the cerebrum and lodged within the cerebellar 

 fossae; (3) the isthmus of the encephalon, the more or less pyra- 

 midal-shaped portion connecting the cerebrum and cerebellum with 

 each other and both with (4) the medulla oblongata. 



