13 



The Nervous System 



The nervous system of the vertebrate consists not only of 

 the brain, spinal cord, peripheral nerves, and autonomic 

 system but also of the various sensory structures: nose, eye, 

 ear, lateral line, etc. This system is composed then of sen- 

 sory cells of one sort or another and conducting cells or 

 neurons. A good part of the substance of the nervous system 

 is made up of supporting tissue which is formed of glial cells 

 and matrix (secreted material). Some scientists have ac- 

 corded an important role to glial cells in the integration 

 and control of central nervous system function, but gener- 

 ally they are viewed as a kind of "insulation." 



The central nervous system is enclosed in connective tis- 

 sue envelopes collectively identified as the meninges; these 

 are highly vascular and share with the central system the 

 spinomeningeal fluid. This fluid fills the central cavity of 

 the brain and spinal cord and also the spaces between these 

 and the meninges. 



THE CONDUCTING AND INTEGRATING SYSTEM 



The nervous system is extremely complex and necessitates 

 an approach from the simple to the more detailed in order 

 to bring the myriad details together in a meaningful way. 

 To achieve this it .seems desirable to describe first the devel- 

 opment of the brain in the mammal, as exemplified by the 

 human, and then to consider the variations of this structure 

 in other forms. 



Mammal 



Embryological development of the brain The invagination 

 of the neural tube has already been described. The brain 

 begins to differentiate by the appearance of three vesicles 

 separated by two constrictions. These divisions are the pros- 

 encephalon, mesencephalon, and rhombencephalon. The 

 third vesicle tapers to the spinal tube (Figure 7-7). 



From the prosencephalon, optic vesicles grow out to 

 either side and these invaginate distally to form optic cups. 

 While the optic cups are forming, bilateral lobes begin to 



grow out from the anterior end of the prosencephalon. 

 These are the beginnings of the cerebral lobes, the begin- 

 nings of the telencephalon (Owen, 1868 — used prosenceph- 

 alon for telencephalon). The prosencephalon has now di- 

 vided into a telencephalon and a diencephalon; the optic 

 cups are attached to the latter. From the roof of the dien- 

 cephalon a small epiphyseal evagination arises. 



While these events are going on, a flexure has developed 

 in the region of the mesencephalon; this is the cephalic 

 flexure. Somewhat later, as the rhombencephalon divides 

 into an anterior metencephalon and a posterior myelenceph- 

 alon, a ponteen flexure develops; the apex of this flexure 

 lies in the region that will become the pons of the adult 

 brain. Between the brain and spinal cord is the cervical, or 

 nuchal, flexure. 



The tubular brain is now five-parted and folded at three 

 points. The walls begin to differentiate, thickening in some 

 spots, thinning in others. The cerebral lobes become pro- 

 portionally very large. Outgrowth of these lobes leaves be- 

 hind a part of the original anterior wall of the prosencepha- 

 lon marked by the anterior commissure and above this the 

 pallial commissure. 



The roof of the diencephalon becomes membranous and 

 in turn highly vascular as a chorioid plexus. The third ven- 

 tricle is constricted posteriorly as the aqueduct of Sylvius. 

 Behind this aqueduct, the fourth ventricle lies within the 

 metencephalon. 



The roof of the myelencephalon is membranous and de- 

 veloped into a chorioid plexus which extends into the fourth 

 ventricle. 



Anatomy of fhe adult mammal brain From this beginning 

 the anatomy of the adult mammal brain can be considered 

 in more detail (Figure 13-1). The cerebral lobes are large 

 and fill much of the cranial cavity in the case of the human, 

 considerably less in some of the lower mammals. The sur- 

 faces of the cerebral lobes are folded into an intricate pat- 

 tern of ridges in some mammals (man) or remain smooth 

 [Ormthorhynchus). Each cerebral lobe (or hemisphere) has a 

 ventricle (cavity) which connects with the prosencoel, or 

 third ventricle, by a foramen of Monroe. These cerebral 

 ventricles extend forward and down into the olfactory bulbs. 



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