.: 



8 CENTRAL NERVOUS MECHANISM. 



a new form of differentiation unknown elsewhere. While the 

 contractility of the amoeba! protoplasm differs but slightly from 

 the contractility of the vertebrate striated muscle, there is an 

 enormous difference between the simple irritability of the amoeba 

 and the complex action of the vertebrate nervous system. Except- 

 ing the nervous or irritable tissues, the fundamental tissues have 

 in ail animals the same properties, being, it is true, more acute 

 and perfect in one than in another, but remaining fundamentally 

 the same. The elementary muscular fibre of a mammal is a 

 mass of differentiated protoplasm, forming a whole physiologically 

 continuous, and in no way constituting a mechanism. Each fibre 

 is a counterpart of all others ; and the muscle of one animal 

 differs from that of another in such particulars only as are 

 wholly subordinate. In the nervous tissues of the higher animals, 

 on the contrary, we find properties unknown to those of the lower 

 ones ; and in proportion as we ascend the scale, we observe an 

 increasing differentiation of the nervous system into unlike parts. 

 Thus we have, what does not exist in any other tissue, a mechanism 

 of nervous tissue itself, a central nervous mechanism of complex 

 structure and complex function, the complexity of which is due 

 not primarily to any mechanical arrangements of its parts, but to 

 the further differentiation of that fundamental quality of irrita- 

 bility and automatism which belongs to all irritable tissues, and to 

 all native protoplasm. 



In the following pages I propose to consider the facts of phy- 

 siology very much according to the views which have been just 

 sketched out. The fundamental properties of most of the ele- 

 mentary tissues will first be reviewed, and then the various special 

 mechanisms. It will be found convenient to introduce early the 

 account of the vascular mechanism, and of its nervous coordinating 

 mechanism, while the mechanisms of respiration and alimentation 

 will be best considered in connection with the respiratory and 

 secretory tissues. The description of the purely motor mechanisms 

 will be brief; and, save in a few instances, confined to a statement 

 of general principles. The special functions of the central nervous 

 system, including the senses, must of necessity be considered by 

 themselves. The tissues and mechanism of reproduction and the 

 phenomena of the decay and death of the organism will naturally 

 form the subject of the closing chapters. 



