Animal Morphology. 123 



As we have arrived towards the central nerve mass, the 

 brain, which is the real seat of the senses, a general outline 

 ought to be given of the metamorphoses and differentiation 

 of this structure and its relation to the senses. 



The question is put, What is brain ? The brain is pro- 

 bably metamorphosed muscle or contractile membrane,* 

 metamorphosed into an impressible concentrating membrane, 

 in contradistinction to a contractile membrane. 



Its proper serous membrane is the arachnoid, with the 

 lining membrane of the ventricles, and its proper mucous 

 membrane is represented in the spreading ossified membrane 

 constituting the skull, segmented separately for each sense 

 and in such a manner as to protect the delicate fabric or 

 contents of all the senses in one centre of co-ordination or 

 reciprocity, and hermetically sealed box. 



The spinal cord, unless viewed as a mere distributive 

 organ to the nerves from the brain, is but an external exten- 

 sion of brain influence, that saves the brain the necessity of 

 continually applying itself to direct and adapt motion once 

 started; for by a series of incident and reflex actions, it 

 sustains the action already started, and, in so far as it 

 continues action once started, it is of inferior order, or has 

 a lower function to perform in the animal economy than 

 the brain itself, to which it stands as clerk of the works 

 during the master's bidding, and sometimes in his absence, 

 or when the brain is having repose. Its bony segmentation 

 is for convenience, and more equal nerve distribution. 



It has a corresponding mucous and serous membrane to 

 that of the brain itself. 



The brain, moreover, is the centre of a series of limbs, 

 whose special functions, by its being an impressible concen- 

 trating membrane, are to co-ordinate and direct motions by a 



* Vide S. W. Mitchell, M.D., U.S.A., "On Injuries of Nerves,' &c. 



