THE STUDY OF ORGANS. 



gets to be performed unconsciously; tlius^for instance, 

 in walking, though every step is a voluntary act, we 

 forget it, and can think of other things as well as if 

 we were standing still. 



In the animals which have the body composed of a 

 series of joints (segmented animals) there is a chain 

 of ganglia all down the body corresponding with the 

 joints, united by nerve cords. The brain is in these a 

 specially large ganglion, or the union of several ganglia 

 modified by their relations with the mouth. In verte- 

 brates, or back-boned animals, which in some respects 

 resemble segmented animals, the brain is much more 

 complicated, and the string of ganglia is replaced by 

 one thick cord; but the latter shows its compound 

 nature by the fact that it gives out at regular inter- 

 vals a pair of branches, one at each side. Bach of 

 these branches springs from a double root; and the 

 anterior, or frout part, of this root contains only motor 

 nerves, and the posterior, or back part, only sensory 

 nerves. 



Nervous tissue is chiefly made of a kind of fat 

 containing a good deal of phosphorus, and almost 

 identical in composition with the yolk of egg. 



The eye arises in the vertebrate embryo as an 

 outgrowth of the brain, supplemented by an ingrowth 

 of epiblast forming the lens. The simplest forms of 

 eye, called ocelli (little eyes), found in the lower 

 invertebrata, consist of a pigment spot, overlying the 

 'termination of a cutaneous uerve. Several diflFefent 

 types of eye are formed among the higher inverte- 



