174 GENEBAL ORNITHOLOGY. 



b. Neurology ; The Nervous System ; Organs of 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 cerehro-spiital nervous 

 system, conformably with which the whole bony framework of the body is erected. A lirain 

 and spinal chord and their lateral prolongations or nerves are tlie ciimmanding superad- 

 ditions, in a vertebrate, to any such uervcjus system as an invertebrate may or does possess. 

 Besides the vertebrate or main nervous system, all brainy veitelirates retain a sympatheiic 

 system of nerves, supposed to represent a modified inheritance of the whole nervous system of 

 Invertebrates. Thus the cerebro-spinal and sympathetic are the two distinct nervous systems 

 of nearly all vertebrates, — 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. Rut the two are inseyiarably 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 Neuramcehm agglutinated by their secretions. Tliey are of two species : 

 Nenramceha cinerea and N. Candida. The former are usually multiradiate, inosculating cells 

 of nerve-substance, which form the "gray matter" of the brain and spinal chord and the 

 (janylia (knots) of nerves; the latter are white, thready, and form the conuecti(ras of the 

 ganglionic masses and the whole substance of ordinary nerve-chords. The gray amojbas are 

 the immediate communicants between the mind and the body of tlie creature ; the wliite 

 amosbas are the mediators between the body and outward things. The gray amosbas translate 

 thought in terms of matter, and conversely ; the white convey the translation. How tliis 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 nervcms system; and, Avith or 

 without the knowledge and consent of the animal, convert these impressions into appropriately 

 responsive actions. This is called the " retlcx action" of the nervcuis system. Some think 

 such reflection is the principal or fmly 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 maiufests 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 wliite 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 im- 

 pulses outward from gray centres, and an afferent function, when they bring impulses in to 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 mi.ved nerves 

 are therefore called se>isori-motor. Thus, each spinal nerve has a posterior sensory ganglion- 

 ated root, and an anterior motor simple root, which soon blend in one clioixl, in which both 

 functions coexist. Some nerves seem to be entirely motor, as those whicli move muscles of the 

 face and tongue. The purest sensory nerves are those of " .spi'cial sense," as the (dfactory, 

 optic, and auditory. Some nerves are so " mixed" as to combine functions of special sense, 

 common sensation, and motion, as that called glosso-])haryngeal, which moves, feels, and 

 tastes. The inotfir effluence of nei-ve tissue upon itself aud other jiarts of the body is literally 

 animation; the sensory influence is nominally malerialisaiion. The physical mechanism of 

 these occult processes in a bird is as follows : — 



