COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 359 



which connect it with the medulla and spinal cord; the brachium pontis or middle peduncle, 

 which joins the two sides of the cerebellum and also conveys tracts between the cerebellum 

 and cerebral cortex; and the anterior peduncles (which were not seen in the dissection) 

 which extend between the cerebellum and midbrain and thalamus. The cerebellum has 

 extensive connections with the area acustica, since the impulses from the ampullae of the 

 semicircular ducts, which are concerned with equilibration, terminate in the area acustica. 



G. SUMMARY OF THE NERVOUS SYSTEM AND THE SENSE ORGANS 



1. The nervous system and the nervous parts of the sense organs are derived from the 

 ectoderm. 



2. The nervous system is subdivided into central, peripheral, and sympathetic nervous 

 systems. The first includes the brain and spinal cord, the second the cranial and spinal 

 nerves, and the third the sympathetic cords, ganglia, and nerves. 



3. The spinal cord consists of a central gray region and a peripheral white region 

 subdivided into tracts. The spinal nerves arise in pairs from the cord at segmental intervals. 



4. Each spinal nerve arises from the cord by two roots, a dorsal and a ventral. The 

 dorsal root is sensory and bears a ganglion composed of sensory cells; the ventral root is 

 motor. The two roots unite to form a spinal nerve. 



5. The spinal nerve very soon divides into a dorsal ramus which passes to the epaxial 

 muscles and adjacent skin, a ventral ramus which passes to the hypaxial muscles and 

 adjacent skin, and a communicating ramus which connects with the sympathetic system. 



6. The ventral rami of the spinal nerves are intricately united by cross-connections in 

 the region of the appendages to form plexi from which the motor nerves to the muscles of the 

 appendages arise. The chief plexi are the brachial plexus to the anterior appendages and 

 the lumbosacral plexus to the posterior appendages 



7. The sympathetic system consists chiefly of a paired ganglionated cord in the mid- 

 dorsal region of the body cavity. These are connected with the spinal and cranial nerves 

 by communicating branches and with the viscera, glands, blood vessels, etc., by means of 

 branches, networks or plexi, and ganglia. 



8. The vertebrate brain consists at first of three vesicles, the forebrain or prosencephalon, 

 the midbrain or mesencephalon, and the hindbrain or rhombencephalon. The first and third 

 soon divide into two vesicles, making five in all. 



9. The five vesicles of the adult vertebrate brain are arranged in a longitudinal series 

 and are named, beginning anteriorly: the telencephalon, the diencephalon, the mesencepha- 

 lon, the metencephalon, and the myelencephalon. 



10. The telencephalon differentiates into the olfactory bulbs, the olfactory tracts, 

 olfactory lobes, and cerebral hemispheres. The hemispheres are lateral expansions of the 

 telencephalon. The olfactory part of the telencephalon is of paramount importance in the 

 lower vertebrates, but later becomes subordinated to the cerebral hemispheres. These 

 latter increase in size in the vertebrate scale until in mammals they cover most of the remain- 

 ing parts of the brain. In mammals the two hemispheres are connected by the corpus 

 callosum. 



n. The diencephalon differentiates into the hypothalamus, the thalamus, and the 

 epithalamus. The hypothalamus includes the optic chiasma, optic nerves and nervous 

 parts of the eye (these are often regarded, however, as belonging to the telencephalon), the 

 infundibulum, and the mammillary body. The infundibulum is an evagination from the 

 floor of the diencephalon; its most ventral portion unites with an outgrowth from the roof 

 of the mouth, the two together forming the pituitary body. The thalamus includes the 



