TRANSACTIONS OF THE SECTIONS. 453 



come extinct. It is known that the ordinary actions of the iris 

 correspond with the impressions of hght on the retina ; and 

 the author has observed that the iris continues to move under 

 the same law after the animal's head has been cut off, or the 

 eye taken out, as long as the retina retains its local sensibility : 

 similar effects take place in other parts of the body. 



The mutual influence of the nerves and spinal marrow seems 

 to be all that is necessary during foetal life, as the absence of 

 the brain in the acephalous foetus does not interfere with any 

 of the functions of the creature until the moment of birth. 



The offices which the coloured substance performs in the 

 nervous system have been matter of speculation with anatomists. 

 One obvious purpose of its existence is to give support and 

 security to the finest subdivisions of the sentient substance ; 

 we therefore find that it affords such protection in proportion 

 to the necessity : hence, in the brain, the coloured substance is 

 soft and tender, while in the ganglia of the nerves it is gene- 

 rally dense and firm. Besides, however, forming a nidus for 

 the ultimate plexuses of the sentient matter, the coloured sub- 

 stance would seem to fulfill some other use not yet ascertained, 

 as wherever it exists it exhibits the same character with respect 

 to colour, varying from yellow to green or brown. Dr. Mac- 

 artney considers the yellow spot in the retina of the human 

 eye, and in that of the monkey and lemur, as a ganglion, having 

 discovered that it contains a more intricate reticulation of the 

 nervous filaments than exists in the other parts of the retina. 



The coloured substances of the nervous system in no degree 

 derive their peculiar tints from the blood that circulates in 

 them, since the colours are palest in the foetus, and grow 

 darker as the nervous system approaches its perfect organi- 

 zation. 



It is a generally received opinion that the ventricles of the 

 brain are cavities or hollow spaces containing some liquid. 

 This error has arisen from the common modes of dissecting the 

 brain, which necessarily separate the surfaces of the ventricles 

 from each other. If, however, the dissection be performed 

 without disturbing the natural position of the parts, not the 

 slightest appearance of cavity or interspace presents itself. 

 The sole use of the ventricles, therefore, seems to be, merely 

 to gain an extent of surface necessary to the development of 

 the peculiar organization of the brain. Apparently there is 

 less superficies in proportion to the magnitude of the mass of 

 the brain in man than in that of animals ; but if we calculate 

 the depth of the surfaces between the convolutions of the cere- 

 brum and on the branches of the arbor vitas in the cerebellum, 



