164 EMBRYOLOGY 



the secondary sympathetic ganglia, which, in turn, connect with the deeper 

 lying primary sympathetic ganglia supplying the viscera. Also supplying the 

 viscera are the nerve fibers which wander in from the vagus nerve. Thus, 

 the internal organs — such as the heart, for example — have a dual innervation. 

 This dual innervation serves to control the rate and the strength of the heart 

 beat. Stimulation of the vagus nerve slows the heart beat, while stimulation 

 of the sympathetic accelerates it. This balanced type of control characteristic 

 of the adult nervous system has its origin in the development of two separate 

 sets of nerve fibers in the embryo. 



The brain and its derivatives 



The development of the brain from the anterior neural plate is the most 

 complicated process in development. When we consider that billions of cells 

 differentiate, forming nerve fibers which run in regular pathways, forming 

 connections with other fibers, we can understand why the factors operating 

 during the development of the higher nervous system are by no means com- 

 pletely understood. The basic principles, however, are likely to be very similar 

 to those governing the differentiation of the spinal nerves and the spinal 

 cord — namely, the directional migration of nerve fibers and the embryonic 

 induction of the differentiation of nerve cells into their specific types. The 

 brain, like all other organs, is dependent to some extent on the development 

 of adjacent organs. 



The early brain after closure of the medullary folds shows division into 

 three. general regions — the forebrain, or prosencephalon; the midbrain, or 

 mesencephalon; and the hindbrain, or the rhombencephalon. The rhomben- 

 cephalon merges with the spinal cord. When the walls of the brain begin to 

 show local differentiation, five primary divisions of the adult brain can be 

 roughly delineated. The differentiations of the walls of the brain involve 

 outgrowths, thickenings, and in some places a thinning of the wall. Con- 

 strictions of the wall also serve to delimit boundaries. These differentiations 

 are shown in Figure 97. 



The first section of the brain, the telencephalon, is bounded by a constric- 

 tion, the velum transversum, and a pit, the optic recess. From the walls of the 

 telencephalon the two cerebral hemispheres develop and connect with each 

 other by means of a commissure which forms in the torus transversus. The 

 cavity of the telencephalon becomes divided into the first and second ven- 



