THE CEREBRO-SPINAL NERVOUS SYSTEM. 499 



will, or of any mental act in the animal, the subject of the experiment, 

 than there is when only the spinal cord is left. The movements are all 

 involuntary and unfelt; and the medulla oblongata has, therefore, no 

 claim to be considered as an organ of the mind, or as the seat of sensa- 

 tion or voluntary poiver. These are connected with parts to be after- 

 wards described. 



C. The Pons Varolii. 



Structure. The meso-cephalon, or pons Varolii (vi., Fig. 341), is 

 composed principally of transverse fibres connecting the two hemispheres 

 of the cerebellum, and forming its principal transverse commissure. 

 But it includes, interlacing with these, numerous longitudinal fibres 

 which connect the medulla oblongata with the cerebrum, and transverse 

 fibres which connect it with the cerebellum. Among the fasciculi of 

 nerve-fibres by which these several parts, are connected, the pons also 

 contains abundant gray or vesicular substance, which appears irregularly 

 placed among the fibres, and fills up all the interstices. The fibres of 

 the facial nerve probably decussate in the pons. 



Functions. The anatomical distribution of the fibres, both transverse 

 and longitudinal, of which the pons is composed, is sufficient evidence 

 of its functions as a conductor of impressions from one part of the 

 cerebro-spinal axis to another. Concerning its functions as a nerve- 

 centre, little or nothing is certainly known. 



An important point in the diagnosis of lesions of the pons Varolii is 

 the occurrence of a variety of what is called crossed paralysis. If the 

 lesion be in the lower half of the pons, there is paralysis, motor and sen- 

 sory, more or less complete of the opposite side of the body, with 

 paralysis of the facial nerve of the same side. If the lesion be in the 

 upper half of the pons, the facial nerve is paralyzed on the same side as 

 the other paralysis, but other nerves are involved. Hyperpyrexia follows 

 after some lesions of the pons. 



D. The Crura Cerebri. 



Structure. The Crura Cerebri (in., Fig. 341) are principally formed 

 of nerve-fibres, of which the inferior or more superficial (crusta) are in 

 part continuous with those of the anterior pyramidal tracts of the medulla 

 oblongata, and the superior or deeper fibres (tegmentum) with the 

 lateral and posterior pyramidal tracts, and with the olivary fasciculus. 

 The middle third only of the crusta contains the fibres of the pyra- 

 midal tracts. The outer third transmits fibres which connect the cere- 

 bellum with the temporal and occipital lobes, and the inner third fibres 

 which connect the frontal lobes with the cerebellum through its superior 

 peduncles. 



