102 THE MECHANISM OF LIFE 



entering the latter via the inferior peduncles. These are the tracts 

 shown in Fig. 28, and called " afferent cerebellar tracts." 



Thus the receptor organs in the trunk and limbs are connected 

 with the ganglia contained in the mid-brain and cerebellum. 

 This is also the case with the great receptors in the head — the 

 visual, auditory, and gustatory organs; with the touch, heat, 

 and cold receptors in the skin of head and face; and with the 

 muscular and articular receptors in the same region, but not 

 with the olfactory receptors. With the latter exception, all the 

 nerve fibres starting from the sense organs in the head and face 

 pass into the medulla via the cranial nerves, and end in separate 

 nuclei. The latter are then connected in complex ways with the 

 corpora quadrigemina, the optic thalami, and the cerebellum. 

 The fibres carrying impulses from the olfactory organs are con- 

 nected directly with the cerebrum ; later on we shall consider the 

 connections of the great sense organs in more detail. 



Connections of the Cerebellum with the Other Parts of the 

 Central Nervous System. — The cerebellum of a higher mammal 

 is connected with the cord, mid-brain, and cortex by three great 

 pairs of tracts contained in the cerebellar peduncles. The 

 inferior peduncles are connected with the white and grey matter 

 of the cord in the way just suggested, the superior peduncles are 

 connected with the mid-brain (mainly corpora quadrigemina 

 and thalami), and the opposite sides of the cerebellum are joined 

 together by the middle peduncles which form what is called the 

 pons varolii. Thus the cerebellum has a grip, so to speak, on 

 the nuclei or ganglia which receive all the sensory impulses 

 coming from every part of the body whatever. Further, since 

 the grey matter of the cord and the nuclei of the cranial nerves 

 are ganglia from which motor fibres go out to the muscles of body, 

 limbs, and head, the cerebellum has also a grip on the centres 

 controlling movements. 



The Connections of the Cortex Cerebri with the Rest of the 

 Central Nervous System. — The cerebral hemispheres are, we 

 have seen, parts of the central nervous system which are super- 

 added to the lower brain, inasmuch as each of them is to be 

 regarded as a great lobe growing out on each side of the original 

 fore-brain vesicle, expanding enormously, and arching over all 

 the other parts of the brain. The base of each hemisphere is 

 formed by the great ganglia called the corpora striata, and these 

 come into close relationship with the ganglia of the fore-brain — 



