CHAPTER IX 



THE MEDULLA OBLONGATA AND CEREBELLUM 



THE brain contains a series of primary sensory and motor 

 centers related to the cranial nerves (see p. 109), the correlation 

 mechanism which serves these sensori-motor centers, and an 

 extensive system of conduction pathways between the brain and 

 spinal cord and between the various correlation centers of the 

 brain itself. 



The brain is divided into two principal parts by a constriction 

 in front of the cerebellum and pons, the isthmus (see p. 122). 

 Above this level lies the cerebrum and below it the rhomben- 

 cephalon, comprising the medulla oblongata or bulb and the 

 cerebellum. The medulla oblongata contains the primary 

 centers concerned with most of the simpler cerebral reflexes, 

 especially those of the visceral, general cutaneous, auditory, and 

 proprioceptive systems (see pp. 112 and 123). The cerebellum 

 is a suprasegmental apparatus developed phylogenetically and 

 embryologically out of the more primitive bulbar nuclei of the 

 vestibular nerve, i. e., out of the acousti co-lateral area of fishes 

 (Figs. 43 and 44, pp. Ill, 112, and Fig. 68). 



The olfactory nerve (I pair), the so-called optic nerve (II 

 pair), and the auditory nerve (VIII pair) are special sensory 

 nerves, whose central connections will be described more in 

 detail below. The remaining nine pairs of cranial nerves of the 

 human body may be briefly summarized as follows: 



The oculomotor nerve (III pair), trochlear nerve (IV pair), and abducens 

 (VI pair) contain the somatic motor fibers and fibers of muscle sense related 

 to the six muscles which move the eyeball. The III pair also contains vis- 

 ceral motor fibers for the ciliary ganglion, from which are innervated 

 the muscles of the ciliary process and iris within the eyeball, i. e., the muscles 

 of accommodation. The trigeminal nerve (V pair) supplies general sensibil- 

 ity to the skin and deep tissues of the face and the motor innervation of the 

 muscles of mastication. The facial nerve (VII pair) innervates the taste- 

 buds of the anterior two-thirds of the tongue (special visceral sensory fibers), 

 the sublingual and submaxillary salivary glands (general visceral efferent 



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