NERVOUS SYSTEM AND BRAIN. 389 NERVOUS SYSTEM. TKe sciliir plrxii^ is thi- ;,'ieat jili'xus of the ab- domen and .suiiplics all the abdoiiiiiial viscera. It is sLiljiliviilcd into sevi'ial smaller ple.xuses, the names of which sulliciently indicate their location and distriliulion. They are known as the aortic plexus, the cadiac ple.xus, tlie phrenic (dia- phragmatic) plexus, the supra-renal ple.xus, the renal plexus, the spermatic plexus, and the superior mesenteric plexus. The hypogastric plexus lies on the front of the sacrum between the two common iliac arteries, liclow the hypo- gastric plexus is continued into the pelvic plexus, which is situated at the side of the rectum and bladder in the male; of the reetvnn, vagina, and bladder in the female. It sends branches to all the pelvic viscera and gives rise to the following subsidiary plexuses, the inferior hemorrhoidal, the prostatic, the vesicle, and in the female the uterine and vaginal. Smaller ganglia are situated in certain viscera, notably in the heart, stomach, intestines, and uterus. They are mostly microscopic in size. All of these ganglia serve as true nerve centres and from them pass otl' branches of distribution to the involuntary muscles of the blood-vessels and of the gastro-intestinal canal, to the thoracic and abdominal viscera, to the secreting cells of the various glands, etc. Functionally the sympathetic system is quite distinct from the cercbro-spinal system. It has been called the system of vegetative e.xistence be- cause of its presiding over the processes of nutri- tion and growth, ihe so-called vegetative func- tions, in contra-distinction to the cerebrospinal system, which presides over such distinctly ani- mal faculties as sensation, motion, and intellect. The sympathetic system controls the so-called automatic mechanisms of the body, the rhythmi- cal beating of the heart, contraction and dilata- tion of the arteries, the peristaltic action of the gastro-intestinal tract, the contraction of smooth muscle in general wherever found, the control of secretion of various glands, etc. It has also been customary to ascribe to the sympathetic a certain 'trophic' function by which was meant the determination of the nutrition of the various tissues and organs. It seems more proper to ascribe this nutritional control to its vaso-motor function, by means of which it determines the blood supply to the various parts. The sympa- thetic nerves contain both alVerent and efferent fibres, so that impulses are transmitted both to and from the ganglionic centres. It is doubtful how many of the afferent impulses are sensory, while certainly many of the efferent impulses are not motor. The close anatomical relation which exists between the cercbro-spinal and the sympa- thetic system has been mentioned, and while the latter seems to be to a certain extent independent in its action, it is not infrequently brought under the control of the former even in our conscious activities. Thus in the case of blushing, an im- pulse reaching the cercbro-spinal centres through the eye. car, or other sense organs caviscs such a reaction on the part of the sympathetic as to bring about a dilatation of the capillaries of the face. Again, in the case of the heart, quick beat- ing and palpitation are frequently caused by the reaction of the sympathetic to impulses which have passed in through ihe cerebro-spinal nerves. BiBLlORRAPllY. Vntemirhiiiiffni iihcr fjchini vnd Riickcnmark des Mensclicn tind tier Siiu- gethiere; Gerlach, The Spinal Cord — Mnnuiil of Bislology. Iiy ^^. .Strieker I F.ng. trans.. New York, 1.S72) ; Golgi. " Sulla struttura della .sos- tanza grigia del cervello," in Gazzetta Mcdica Italiuna-Loinbdida, vol. vi. (1S73): Golgi, HuUa fiiia analoiiiiu dci/li ortjuni ceiitrali del sislciiia nervosa {Reggio-Kmilia. l.S^i2). For review of the liner structure of the neurone, consult The Nervous Nyslem and Its l!ons1Huent yeurones, by Lewellys F. Barker ( Xew York, 1 SO'J ) . For gross anatomy of nervous system, consult: Gray's Anatomy, 14th edition; Quain's Anatomy, vol. iii., parts 1 and 2; Gardinier, The Anatomy of the Central Xerrous t<ysleni. For histology of the nervous system, consult: .Stolir, Text-boul; of /jf ts?o/o3y (Philadelphia, 1901); Piersol, Xornial Hisloloyy : Schiifer, .4 Course of Practical His- toloyy and Essentials of Histology. NERVOUS SYSTEM, Evolution of the. The nervous system com|)rises those tissues that have the property of irritability or of sensitive- ness to external inllucnces, and whose function it is to mediate between the outer world and the inner, and to convey to the nuiscles and other organs motor impulses so as to bring about move- ments harmonious and advantageous to the or- ganism. In the nervous system also are centred those phenomena which we designate as thought, reason, will, etc. The origin of the nervous sys- tem is apparently independent of that of the muscular system, but the development of nerv- ous tissue accompanies that of the contractile organs. In the most primitive organisms, such as the amoeba, the whole protoplasm of the ani- mal body is .sensitive to external inlluences; there is a diffuse irritability. Thus the whole body of the amoeba moves away from strong light. Moreover, it has the ability to move with refer- ence to objects that stimulate it in some physical or chemical way. Lower Ixvertebk.te.s. In the Metazoa the nervous system develo])s in the ectoderm or outer layer of the body wall, which is the region that comes in contact with the outside world and hence is the one in which we should expect sensi- tiveness to become first developed and localized. Some of the coelenterates have in the ectoderm superficial sensory cells with which the nerve cells are in contact. Fibres from the nerve cells connect in turn with the fibrillar jiart of the muscle cells. In the llatworms the nervous sys- tem is almost com])letely separated from the epithelium and lies in or clos(> under the dermal musculature, and a "brain' is best developed in some of the Polycladida and some trematodes. The nervous system of echinoderms has a devel- opment unknown in other animals. It is com- posed of three independent systems: (I) the su- perficial oral; (2) the apical oral; (.3) the deeper oral. The superficial oral consists of a ring around the oesophagus, and of radial nerves which branch out from this ring to the arms and radii. This .system innervates the intestine, am- bulacral feet, and integument. The nervous sys tern of ilollusca is of a complex type of its own, but shows some affinities to that of annelids and arthro))ods. In its simplest form (chitons) it consists of a cerebral part, from which four pairs of commissures are sent off. Passing ventrally from it there is a ring with ganglia, and passing backward two pairs of conunissures. one called 'pedal" and the other 'pallial.' All of these con- tain ganglion cells along their whole length, and
