Xin. THE COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 



AND THE SENSE ORGANS 



A. GENERAL CONSIDERATIONS 



In this section of the manual we shall include both the nervous system proper and the 

 sense organs. The former functions for the conduction, co-ordination, and correlation of 

 stimuli, while the latter is differentiated for the reception of stimuli. 



1. The parts of the nervous system. The nervous system consists of three parts: the 

 central nervous system, the peripheral nervous system, and the sympathetic system. The 

 central nervous system is composed of the brain, situated within the skull, and of the spinal 

 cord, situated within the neural canal formed by the neural arches of the vertebrae. Brain 

 and spinal cord are made up of both nerve-cell bodies (gray matter) and of nerve-cell processes 

 (white matter). The peripheral nervous system consists of the cranial nerves springing from 

 the brain, and the spinal nerves, springing from the spinal cord. The nerves are markedly 

 metameric in arrangement, a pair being typically present for each segment. Nerves are 

 composed of nerve-cell processes only. The sympathetic nervous system controls and regulates 

 in general the involuntary activities and the organs which subserve those functions, as the 

 heart, the digestive tract, the smooth musculature in general, the secreting glands, blood ves- 

 sels, respiratory and reproductive systems. It consists chiefly of a paired cord lying against 

 the dorsal wall of the coelom from which branches and networks extend into the viscera. The 

 sympathetic system is connected with the peripheral nervous system of which indeed it is 

 an outgrowth. Associated with both of these systems are numerous ganglia, often segmen- 

 tally arranged. A ganglion is a collection of nerve-cell bodies, situated outside of the central 

 nervous system. Within the central nervous system a similar collection is called a nucleus 

 or center, although a few of these are designated ganglia, as retentions from an older termi- 

 nology. 



2. The development of the nervous system. The central nervous system is formed as 

 previously learned by the infolding of the ectoderm in the median dorsal line of the embryo. 

 A tube extending the length of the embryo is thus produced. That portion of the tube 

 situated within the head develops into the brain, while that portion posterior to the head 

 becomes the spinal cord. The originally single layer of enrolled ectoderm cells proliferates to 

 form a thick zone of cells around the central cavity. Most of these become nerve cells, 

 while the remainder give rise to supporting cells. In the spinal cord of all vertebrates and 

 in the brains of the lower ones the zone of nerve cells retains its primitive position around 

 the central cavity, but in the higher vertebrates there is considerable migration of nerve cells 

 to the periphery of the brain. Axones and dendrites arise from the nerve cells by outgrowth; 

 part of these remain in the central nervous system forming a peripheral zone of fibers, called 

 the white matter. Part of the processes of the nerve cells grow out from the central nervous 

 system (or grow into it from sense organs and ganglia) forming the nerves or peripheral 

 nervous system. The ganglia of the peripheral nervous system arise chiefly from the neural 

 crests; these are a pair of longitudinal cords of ectodermal cells which are left outside of the 

 neural tube at the time of its closure. The sympathetic ganglia arise by the migration of 

 cells from the neural tube or from the ganglia of the peripheral nervous system. 



In the development of the brain the original simple tube is first marked off into three 

 vesicles, the primary brain vesicles, by two constrictions (Fig. 67). These visicles are named 



296 



