SEC. IV ANATOMY OF BIRDS 257 



h. Neurology; The Nervous System; Organs op Special 



Senses 



The Nervous System of any Vertebrate determines the form of 

 such an animal ; in fact, the beautiful skeleton we have examined 

 is simply a sketch in bone of the cerebrospinal nervous system, con- 

 formably with which the whole bony framework of the body is 

 erected. A brain and spinal cord and their lateral prolongations 

 or nerves are the commanding superadditions, in a vertebrate, to 

 any such nervous system as an invertebrate may or does possess. 

 Besides the vertebral or main nervous system, all brainy vertebrates 

 retain a sympathetic system of nerves, supposed to represent a modified 

 inheritance of the corresponding nervous system of Invertebrates. 

 Thus the cerebrospinal and sympathetic are the two distinct nervous 

 systems of all vertebrates which have a skull and brain. The 

 former presides over the animal life of the creature, — its sensations, 

 perceptions, and voluntary actions ; the latter more especially over 

 its vegetative functions, as digestion, respiration, circulation, and 

 reproduction, which are more or less involuntary. But the two are 

 inseparably connected, anatomically and physiologically, so that no 

 distinct line can be drawn between them. Nerve-tissue consists of 

 an aggregation of nerve-cells and their investing substance, — the 

 bodies of a myriad Neuramoebce agglutinated by their secretions. 

 They are of two species : Neuramxha cinerea and N. Candida. The 

 former are usually multiradiate, inosculating cells of nerve-substance, 

 which form the " gray matter " of the brain and spinal cord and 

 the ganglia (knots) of nerves ; the latter are white, thready, and 

 form the connections of the ganglionic masses and the whole sub- 

 stance of ordinary nerve-cords. The gray amoebas are the imme- 

 diate communicants between the mind and the body of the creature ; 

 the white amoebas are the mediators between the body and outward 

 things. The gray amoebas translate thought in terms of matter, 

 and conversely; the white convey the translation. How this is 

 done, no one knows, but the fact is manifest. In ordinary language, 

 gray nerve centres receive from white tracts impressions made upon 

 the periphery of the nervous system; and, with or without the 

 knowledge and consent of the animal, convert these impressions 

 into appropriately responsive actions. This is called the "reflex 

 action " of the nervous system. Some think such reflection is the 

 principal or only activity of the nerve-tissue, taking animals to be 

 mere automata, the mechanism of which is only set in motion by 

 external stimulation. Others think that animals, and even human 

 beings, have in their consciousness an inner spring of action, vaguely 

 called "spiritual," whose operations upon the matter of their bodies 



s 



