258 GENERAL ORNITHOLOGY part ii 



manifests what is called by some "mind," by others "soul." I am 

 satisfied of the correctness, in the main, of the latter view ; but, 

 however this may be, it is quite certain that white nerve-tissue is a 

 means of carrying something to and fro, which something is called 

 a "nerve impulse," for want of knowing what it is. White nerves 

 have therefore an efferent function, when they carry impulses out- 

 ward from gray centres, and an afferent function, when they bring 

 impulses into gray centres. The former is their motor function ; the 

 latter is their sensory function. In nerves at large, impulses of both 

 kinds travel in the same tracts without interference ; such mixed 

 nerves are therefore called scnsorbnotor. Thus, each spinal nerve 

 has a posterior sensory ganglionated root, and an anterior motor 

 simple root, which soon blend in one cord, in which both functions 

 coexist. Some nerves seem to be entirely motor, as those which 

 move muscles of the face and tongue. The purest sensory nerves 

 are those of " special sense," as the olfactory, optic, and auditory. 

 Some nerves are so "mixed" as to combine functions of special 

 sense, common sensation, and motion, as that called glossopharyn- 

 geal, which moves, feels, and tastes. The motor effluence of nerve- 

 tissue upon itself and other parts of the body is literally animation ; 

 the sensory influence is nominally materialisation. The physical 

 mechanism of these occult processes in a bird is as follows : — 



The Brain (Lat. cerehrum ; Gr. lyKkc^aXov, egkephalon, the enceph- 

 alon) is the anterior dilatation and complication of the main nervous 

 axis of the body, contained within the skull. It resembles a soap- 

 bubble blown at the end of a pipe, being not less beautiful in its 

 iris-quality, and not less lasting. It is primarily triune, or three- 

 fold, beginning as three such bubbles, called the anterior, middle, and 

 posterior cerebral vesicles, corresponding to what are afterward the 

 fore-brain, mid-brain, and hind-brain, or prosencephalon, mesencephalon, 

 and opisthencephalon. Two of these primitive vesicles subdivide each 

 into two, so that five segments of the brain result. These five are 

 commonly called prosencephulon, diencephcdon, mesencephalon, epen- 

 cephalon, and metencephalon, named from before backward ; and they 

 respectively corresjiond with parts of the adult brain known as 

 cerebrum proper, optic thalami, optic lobes, cerebellum, and medulla 

 oblongata. The birth and multiplication of gray neuramoebas cause 

 thickenings of the bladdery membranes in various places and ways ; 

 all such gray deposits are the ganglia of the brain, and the great 

 peripheral ganglion is the cortical layer or "bark of the brain." 

 Similar deposits of white neuramoebas connect all these ganglionic 

 colonies, furnishing various commissures of the brain. The cavity 

 of the original bubbles, continuous with the hollow of the pipe-stem 

 or spinal cord (which was at the outset a furrow along the back of 

 the embryo, not a tube) becomes partially divided up into several 



