594 



VERTEBRATE LIFE AND ORGANIZATION 



Lortgitadinal 

 fissui^e oF 

 Cerebrum 



Optic chiasma- 



Rhinal fissure- 



La.tei'a.1. 

 f issui'e 



Optic tract 



Hypothcdamus 



Meszncephalon^ 



Tri^i 



eminal 



ncx-ve" 



Abducens nerve k:;> 

 Acoustic nerve- 



GlossopHaryn^eal 

 nervz' 



VaOuS nerve 

 Hypoglossal nerve 



Ventral median -Fissure 



actory bulb 

 Optic nerve. 



Cerebral 

 hemisphere 



Pituitary 

 Stalk 



Oculomotor 

 nci've 



Trochlear 

 nerve 



Pons 



:iW''-' 



Y/l 



Facial nerve 

 cIluiTi. 



O^ 



■Medulla oblongata 



^j — Spinal eiccessory nerve. 



Figure 29.11. A ventral view of the brain of a sheep. The stumps of all but the 

 first pair of cranial nerves are visible. The olfactory nerves consist of the processes of 

 olfactory cells (cf. Fig. 29.1 £), which enter the olfactory bulbs in many small groups 

 that cannot be seen with the unaided eye. The rhinal fissure separates the ventral, 

 olfactory portion of each cerebral hemisphere from the rest of the hemisphere. The 

 paths of the optic fibers in the optic chiasma have been indicated by broken lines. 



organs; to the small muscles associated with the hairs; to the ciliary and 

 iris muscles in the eye; and to many of the glands of the body (Fig. 

 29.12). These efferent fibers constitute the autonomic nervous system. 

 The organs supplied by these fibers function automatically, requiring 

 no thought on our part. Indeed, they cannot be controlled voluntarily. 

 It should be emphasized that the autonomic nervous system is by defini- 

 tion a motor system, and the afferent fibers that return from internal 

 organs are not a part of this system, even though they may be in nerves 

 composed largely of autonomic fibers. 



The autonomic nervous system is morphologically unique in that 

 the autonomic neurons that emerge from the central nervous system do 

 not extend all the way to the effectors, as do other efferent neurons. 

 They go only to a peripheral ganglion in which there is a relay, and a 

 second set of autonomic fibers continues from the ganglion to the organ. 

 Autonomic fibers having their cell bodies in the central nervous system 

 and extending to a peripheral ganglion are known as preganglionic 

 fibers; those having their cell bodies in the ganglia and extending to the 

 organs are the postganglionic fibers. 



The autonomic nervous system is subdivided into sympathetic and 

 parasympathetic systems. Most organs innervated by the autonomic 

 nervous system receive fibers of both types. The preganglionic sympa- 



