THE PONS VAEOLII. 307 



operation after the excision of the brain. As with the spinal cord, so 

 with it : its simple action may continue though its commissural action 

 has ceased, and this either through conditions of disease or by the ad- 

 ministration of drugs. In lesions of the brain respiration may still con- 

 tinue, as it may also when sensation and voluntary motion have been ar- 

 rested by the breathing of chloroform. 



OF THE PONS VAKOLH. 



The pons varolii consists of a loop of fibres passing from one crus 

 cerebelli to the other, around the tracts of communication structure of the 

 between the cord and the brain. As shown in Fig. 145, P ns varolii. 

 they do not form a continuous superficial commissure, but, at a certain 

 distance below, interlace with the fibres of the pyramids ; moreover, 

 among their deeper fibres gray vesicular matter occurs. That they con- 

 stitute mainly a commissure for the cerebellum is apparent from the cir- 

 cumstance that, in those animals which have the median cerebellar lobe 

 only, there is no p'ons, and in other cases its relative magnitude is in 

 proportion to the size of the cerebellar hemispheres. 



FUNCTIONS OF THE PONS VAKOLH. 



The functions of the pons varolii are therefore twofold: it acts as a 

 conductor, and also as a nerve centre. In the first respect, it Functions of 

 is the channel from the spinal column to the cerebrum and the P ns - 

 cerebellum, and also between the cerebellar halves, and experiments upon 

 it, in giving rise to sensations and motions, are in conformity with what 

 we should anticipate from the structure and functions of the spinal cord. 



In the second respect, as a nervous centre, it has been stated that, when 

 the cerebrum and cerebellum are removed, but the pons left untouched, 

 an animal gives tokens of sensation when pinched or irritated, and like- 

 wise executes motions which have an object ; these, however, were no 

 longer observed after the removal of the pons. 



We have had repeated occasion already to mention that the surest 

 guide which can be followed in interpretations of the func- 



. (. , , . T ^ Dr. Carpen- 



tions of the nervous system is comparative physiology. Our te r's views of 

 views of the action of the spinal cord, medulla oblongata, J^^H!" 

 and even portions above, hereafter to be described, will be nai cord of ver- 

 rendered clear by a knowledge of the structure and func- jeentrai d 

 tions of the ventral cord of the articulata, the analogy of cord of articu- 

 which to the parts we have had under consideration was { 

 first correctly pointed out by Dr. Carpenter. I therefore transcribe from 

 his General and Comparative Physiology the following paragraphs, which 

 present his views with perspicuity. 



" The plan on which the nervous system is distributed in the sub- 



