SENSE ORGANS AND NERVOUS COORDINATION 597 



dendrites and cell bodies of the efferent neurons. The gray commissure 

 is composed of fibers crossing from one side of the spinal cord to the 

 other. The gray matter lying dorsal to the central canal is concerned 

 with relaying sensory impulses that enter the cord, and the part lateral 

 and ventral to the central canal relays motor impulses that leave the 

 cord in the efferent neurons. 



Much of the white matter consists of the fibers of afferent neurons, 

 some of which extend some distance in the central nervous system before 

 entering the gray matter, and of afferent internuncial neurons which 

 end in the brain. The rest of the white matter consists of the processes 

 of efferent internuncial neurons coming from the brain to the efferent 

 neurons. All afferent impulses that enter the spinal cord cross to the 

 opposite side before they reach the brain, and efferent impulses coming 

 from the brain cross within the brain. Thus afferent impulses initiated 

 on the left side of the body reach the right side of the brain, and efferent 

 impulses initiated in the right side of the brain reach the left side of 

 the body. 



Though all of the white matter looks the same, careful experimenta- 

 tion has enabled neuroanatomists to localize the various groups of fibers 

 that comprise it. Impulses initiated by temperature receptors on the left 

 side of the body, for example, are carried to the brain by fibers located 

 in the lateral portion of the white matter on the right side of the cord 

 (Fig. 29.8). A lesion in this part of the cord would prevent one from 

 being conscious of temperature changes on the opposite side of the body 

 posterior to the lesion, though one would still respond reflexly to such 

 changes. 



The Brain. Major Parts of the Brain. A brief consideration of 

 the embryonic development of the brain makes it easier to understand 

 its major divisions and parts. The brain develops as a series of enlarge- 

 ments of the anterior portion of the embryonic neural tube (Fig. 29.13). 

 In an early embryo, there are only three swellings (a forebrain, mid- 

 brain and hindbrain), but the forebrain and hindbrain are later sub- 

 divided, so five regions are present in an adult. The forebrain divides 

 into a telencephalon and a diencephalon. The telencephalon differen- 

 tiates into a pair of olfactory bulbs, which receive the endings of olfactory 

 cells, and a pair of cerebral hemispheres. The lateral walls of the 

 diencephalon become the thalamus, its roof the epithalamus, and its floor 

 the hypothalamus. Fibers in the optic nerves cross below the hypothalamus 

 and form an optic chiasma (Fig. 29.11). All of the optic fibers cross and 

 go to the opposite side of the brain in most vertebrates, but only half 

 of them cross in mammals. The pituitary gland is attached to the hy- 

 pothalamus just posterior to the chiasma, and the pineal body is attached 

 to the epithalamus. No further division occurs in the midbrain, or 

 mesencephalon, but its roof differentiates into a pair of optic lobes in all 

 vertebrates. In addition to the optic lobes, or superior colliculi, the 

 mesencephalic roof of mammals bears a pair of interior colliculi. The 

 hindbrain divides into a metencephalon, the dorsal portion of which 

 forms the cerebellum, and a myelencephalon, which becomes the me- 

 dulla oblongata. 



