RESPONSIVENESS IN VERTEBRATES 



The organ systems related to metabolism perform certain functions which 

 are necessary if individual cells are to remain alive. They deliver food and 

 oxygen to all cells and remove their waste products, providing a suitable 

 internal environment for maintenance and function of the diflferent tissues. 

 The stability of the internal environment, upon which our feeling of well- 

 being depends, requires constant adjustment of the interlocking functions. 

 The many systems must work together, or be coordinated. If any system 

 concerned with the metabolic needs of cells ceases to function, the organism 

 cannot remain alive; it becomes abnormal if the activities of its organs are 

 not correlated in the usual way. 



Although we are not conscious of the regulation of our internal environ- 

 ment, we are well aware of many of the adjustments we necessarily make to 

 our external environment. Every mechanism of regulation, adaptation, or 

 coordination, whether conscious or unconscious, is possible because proto- 

 plasm has the capacity of responsiveness; a cell responds by internal reaction 

 to a stimulus, or change in its environment. There are two ways of altering 

 the immediate environment of cells in the vertebrate body. One is by means 

 of impulses that pass along the nerves which penetrate to every part of the 

 animal, and the other is by means of substances that circulate in the blood. 

 Nervous coordination is brought about by the activities of the sense organs 

 and nervous system. The secretions which enter the blood from the endocrine 

 or ductless glands make what is known as chemical coordination possible. 



We shall first review the structure of the systems related to coordination 

 and then explain the ways in which they function. These systems are the 

 nervous system, together with the organs of special sense, skeletal system, 

 muscular system, and endocrine system. The skeletal and muscular systems 

 are discussed here because so many adaptive responses to the external en- 

 vironment involve bodily movements for which the nervous, muscular, and 

 skeletal systems are responsible. 



Organ Systems Related to Coordination 



The Nervous System. The nervous system of vertebrates is divided for 

 purposes of discussion into two parts: the central nervous system and the 

 peripheral nervous system. The central nervous system is composed of the 

 brain and spinal cord. The peripheral nervous system consists of the nerves 

 which connect the brain and spinal cord with all parts of the body. 



The Central Nervous System. The central nervous system develops in the 

 same way in all vertebrates. Soon after its first appearance it is found to have 

 five regions in the brain, which can be distinguished from the spinal cord. 

 These five regions are known, from anterior to posterior, as the telencephalon, 

 diencephalon, mesencephalon, metencephalon, and myelencephalon (see Fig. 

 5.21, p. 156). None of these parts is lost in any vertebrate, but differences 

 in the degree of development of certain regions, especially of the telencephalon 



87 



