goo VERTEBRATE LIFE AhlD ORGANIZATION 



afferent internuncial neurons. These are sensory nuclei, just as the dorsal 

 columns of the cord are sensory columns. The ventral nuclei contain 

 the cell bodies of the efferent neurons of the cranial nerves, and hence 

 are motor nuclei. In mammals, reflexes that regulate the rate of heart 

 beat, the diameter of arterioles, respiratory movements, salivary secre- 

 tion, swallowing and many other processes are mediated by these nuclei. 

 Afferent impulses come into the sensory nuclei, are relayed by the inter- 

 nuncial neurons to the motor nuclei, and efferent impulses go out to 



the effectors. 



Cerebellum and Pons. Motor and sensory nuclei associated with 

 cranial nerves are also found in the metencephalon and mesencephalon 

 and other reflex arcs involve these regions. All vertebrates have a cere- 

 bellum, which develops in the dorsal part of the metencephalon, and is 

 a center for balance and motor coordination. Impulses from the parts 

 of the ear concerned with equilibrium, from the lateral line (if present), 

 and from the proprioceptors in the muscles of the body enter it. It is 

 small in many of the lower vertebrates such as the frog (Fig. 21.18), in 

 which muscular movements are not complex, but it is very large in birds 

 and mammals. The mammalian cerebellum has neuronal connections 

 with the cerebral hemispheres, and many motor impulses initiated in 

 the cerebral hemispheres pass through the cerebellum for final integra- 

 tion with respect to the position of the body and degree of contraction 

 of the muscles before going to the motor nuclei and columns. Much of 

 the gray matter of the mammalian cerebellum lies on the surface, where 

 there is more room for the increased number of cell bodies. The surface 

 is also complexly folded, which further increases the surface area avail- 

 able for cell bodies. 



The floor of the metencephalon is unspecialized in lower vertebrates, 

 but this region differentiates into a pons in mammals (Figs. 29.11 and 

 29.14). Evolution of the pons is correlated with the elaboration of the 

 cerebellum. It contains nuclei that relay cerebral impulses into the cere- 

 bellum, and transverse fibers that interconnect the two sides of the 

 cerebellum. 



Optic Lobes. In fishes and amphibians, the optic lobes (Fig. 21.18) 

 receive impulses not only from the eyes, but also from many of the other 

 sense organs. This sensory information is integrated, and motor impulses 

 are sent to the appropriate efferent neurons. The optic lobes are the 

 master integrating center of the brain, in so far as these vertebrates have 

 such a center. The cerebral hemispheres of the lower vertebrates are 

 concerned almost exclusively with integrating olfactory impulses. In 

 reptiles, other sensory data are sent to the cerebral hemispheres, and 

 they begin to assume some of the functions of the optic lobes. Still more 

 sensory information is sent to the cerebral hemispheres of birds and 

 mammals, and the hemispheres of mammals have taken over most of 

 the functions of the optic lobes. The optic lobes (superior colliculi) 

 of mammals (Fig. 29.14) remain as relatively small centers for pupillary 

 and other optic reflexes. A pair of inferior colliculi are present posterior 

 to them, and they are a center for certain auditory reflexes. 



Thalamus and Hypothalamus. The thalamus is a relay center to 



