506 Animal Biology 



and as pathways to and from the brain. The white matter of the cord 

 has ( 1 ) long ascending tracts to transmit afferent impulses from the spinal 

 nerves to the brain and (2) long descending tracts to transmit efferent 

 impulses from the motor centers of the brain to the anterior columns of 

 the cord to control muscular movements. 



The autonomic nervous system consists of ( 1 ) the sympathetic (tho- 

 racolumbar) which has centers, ganglia, and plexus in the cervical, tho- 

 racic, and lumbar regions of the spinal cord, (2) the parasympathetic 

 (craniosacral) which consists of centers and ganglia of the cranial and 

 sacral parts of the autonomic system, and ( 3 ) the enteric which consists 

 of the part of the autonomic system associated with the walls of the 

 alimentary tract. The autonomic nervous system innervates smooth 

 muscles, cardiac muscles, and glands. The autonomic system is a highly 

 important functional portion of the entire nervous system and not a self- 

 controlling, independent, segregated unit, as the word autonomic might 

 imply. In fact, the autonomic is one of the most essential parts of our 

 vital nervous system. 



IX. ENDOCRINE (DUCTLESS GLAND) SYSTEM OF MAN 



The structure and functions of various organs in the human body are 

 also affected by substances produced in other organs and transmitted 

 primarily by the blood. This chemical coordination is brought about by 

 specific chemical substances known as hormones (hor' mon) (Gr. hor- 

 maein, to excite). These hormones are manufactured in certain organs 

 from ingredients brought to them by the blood and carried away without 

 the benefit of ducts. The more important ductless glands and their hor- 

 mones and functions are given in summaries. 



Endocrine glands (en'dokrin) (Gr. endon, within; krinein, to sep- 

 arate) and their secretions which contain the specific hormones are in- 

 fluenced by such factors as ( 1 ) the quantity and quality of foods brought 

 to them by the blood, (2) the action of hormones from other endocrine 

 glands, and (3) the action of certain parts of the nervous system such as 

 the hypothalamus of the brain, the sympathetic nervous system, etc. 

 Endocrine glands were studied separately, but recent work has shown 

 the great interdependence of many of them, and this new approach has 

 been profitable in getting a more correct picture of them. Regulating 

 substances in invertebrate animals are probably present, but their roles 

 and distribution are not well known. For example, it is probable that 



