96 ANATOMY AND PHYSIOLOGY. 



action. Tickling of the soles of the feet, causing the action of the 

 diaphragm that takes place in laughing, tickling of the throat caus- 

 ing vomiting, &c., are examples of a similar connexion. 



A subject of the utmost interest to the physiologist is presented 

 in the modifications which the corresponding parts of the nervous 

 system undergo in the different classes of animals. In none of the 

 lowest tribes of the Radiata have any traces of a nervous system 

 been discovered, though these creatures seem to possess both feel- 

 ing and voluntary powers. In the long round worm which infests 

 the human intestines, a slender nervous filament passes along the 

 lower part of the belly, and is divided by the gullet into two 

 branches. The nervous filaments in the star-fish encircle the 

 mouth, and radiate to its five divisions. In the Articulata the 

 nervous cords are interrupted by knots or ganglia, which, it is proba- 

 ble, perform functions analogous to the brain and spinal marrow of 

 the Vertebrata. The nervous system of the Mollusca contrasts with 

 that of the Articulata, in assuming more of a circular form. In 

 that of the sepia, there is a large ganglion, which is enclosed in 

 something like a rudimentary cranium, and probably performs func- 

 tions analogous to those of the brain. The parts from which the 

 optic nerves are derived in this animal, are even larger than the part 

 representing the brain. 



In the vertebrated division, a brain and spinal marrow are always 

 present, but the size of the parts composing the brain especially, is 

 relatively so much altered, as almost to prevent them from being 

 recognized. Among animals of this division, fishes have the most 

 simple nervous system. From these there is a regular gradation in 

 complexity of organization up to man, in whom all the parts be- 

 longing to the other classes are found, besides some that are pecu- 

 liar to himself. 



The nervous system of man is particularly distinguished by the 

 ample development of the cerebral hemispheres. The human cere- 

 brum extends so far backwards as to cover the whole of the cere- 

 bellum ; the ourang-outang's cerebrum allows the cerebellum to be 

 seen behind it, and the otter's and sheep's do so still more de- 

 cidedly. In the marmot, and other Rodentia, not only the cere- 

 bellum, but also the parts from which the optic nerves arise (optic 

 tubercles, also called corpora quadrigemina) are partially exposed^ 

 and the convolutions on the surface of the brain have disappeared. 

 In birds the exposure is still greater, and becomes complete in rep- 

 tiles and in fishes. 



