158 FUNCTIONS OF THE NERVOUS SYSTEM. 



the functions of the muscles controlling the various orifices are those most 

 affected ; and it is by the spasms affecting the organs of respiration or deglu- 

 tition that life is commonly terminated. Various remedial agents will probably 

 be found to operate, by occasioning increased excitability in some particular 

 segments of the Cord ; so that the usual stimuli applied to the parts connected 

 with these, will occasion increased muscular action. This seems to be the 

 case, for example, in regard to the influence of aloes on the rectum and uterus, 

 cantharides on the neck of the bladder and adjoining parts, and secale cornu- 

 tum on -the uterus. The mode of influence of cantharides is illustrated by a 

 curious case, related by Dr. M. Hall, of a young lady who lost the power of reten- 

 tion of urine in consequence of a fatty tumour in the spinal canal, which 

 gradually severed the Spinal Cord, and induced paraplegia. The power of 

 retaining the urine was always restored for a time by a dose of tincture of 

 cantharides, which augmented the excitability of the segment of the cord, 

 with which the sphincter vesicae is connected. The researches of Valentin, 

 when grafted (as it were) on the doctrines of Dr. M. Hall, afford the key to 

 the explanation of the numberless sympathetic influences of the organs of 

 nutrition, &c., upon one another ; by showing that they are all connected with 

 the Spinal Cord ; and that the muscular structure, with which they are all 

 provided, may be excited to contraction through it. And, lastly, t % he more 

 recent observations of Dr. M. Hall, in regard to the peculiar excitor power 

 that belongs to the nervous fibres distributed on various serous and fibrous 

 membranes, will probably lead, when they have been fully carried out, to the 

 explanation of the various convulsive actions that result from pressure or 

 irritation affecting these parts. 



XIII. Comparative Anatomy of the Encephalon. 



213. The assistance which the Physiologist has hitherto derived from the 

 study of the Comparative Anatomy of the Encephalon in Vertebrata, is not so 

 great as might have been expected ; there can be little doubt, however, that 

 much is yet to be learned from it. Certain general inferences appear well 

 established; and it is chiefly in questions of detail that difficulties still exist. 

 The Encephalon may be described as consisting of the Cerebral Hemispheres, 

 the Cerebellum, and the Medulla Oblongata with its chain of ganglia. The 

 relative proportion of the two former to the latter is such in Man, that their 

 character would not be readily understood by the inspection of his Brain alone ; 

 and it is one of the most interesting results of the comparison of it with the 

 brains of animals of the inferior tribes that the great change which we there 

 find in the proportion of the parts, makes evident the importance of what 

 would have been otherwise considered subordinate appendages. This is 

 peculiarly the case in Fishes. There may be noticed in the Encephalon of 

 that class four distinct ganglionic enlargements; of which the posterior is 

 usually on the median line, whilst the others are in pairs. The posterior, 

 from its position and connections, is evidently to be regarded in the light of a 

 Cerebellum; and it bears a much larger proportion to the rest, in this class, 

 than in any other. The pair in front of this are not the hemispheres of the 

 Cerebrum, as their large size in some instances (the Cod for instance) might 

 lead us to suppose ; but they are immediately connected with the Optic nerve, 

 which, in fact, terminates in them, and are therefore to be considered (like 

 the chief part of the cephalic masses of In vertebrated animals) as Optic Ganglia. 

 In front of these are the Cerebral Hemispheres, which are small, generally 

 destitute of convolutions, and possess no ventricle in their interior, except 

 in the Sharks and Rays, in which they are much more highly developed than 

 in the Osseous Fishes. Anterior to these is another pair of ganglionic 



