30 PHYSIOLOGY [NERVOUS the cervical spinal cord through the last cervical and first thoracic ganglia of the sympathetic. Stimulation of these accelerator nerves causes a quickening of the heart s beat, in which, however, "what is gained in rate is lost in force" (Foster). They are referred to here as showing another kind of nervous action, quite different from inhibition. Thus any nerve-centre concerned in reflex movements may have, by impulses reaching it from the periphery, its action inhibited or restrained or accelerated. Nerves Influence of Nerves on Blood -Vessels. If the sympathetic nerve and be divided in the neck, there is a dilatation of the vessels and an blood- increase of temperature on the same side ; but irritation by weak vessels, induction -currents of the cephalic end will cause the vessels to contract and the temperature to fall. In the sympathetic, there fore, there are nerve-fibres which influence the contractile coats of the blood-vessels. These fibres, called "vaso- motor," originate from a vaso-motor centre in the medulla oblongata between the point of the calamus scriptorius and the lower border of the corpora quadrigemina, in the floor of the fourth ventricle. From this chief vaso-motor centre nervous influences emanate which tend to keep the smaller vessels in a more or less contracted condition. If it be injured, paralysed, or destroyed there is at once great dilatation of the vessels, more especially those in the abdominal cavity, and the blood collects in these dilated vessels. This of course dimi nishes the arterial pressure in the larger vessels. Consequently, by observations on blood - pressure, it has been found possible to study the conditions of vaso-motor action. By connecting a kymo graph (a recording manometer) with a large vessel, say the carotid, observing for a time the mean blood - pressure, and afterwards injuring the supposed vaso-motor centre, Ludwig and his pupil Owsjannikoff at once observed an enormous fall of blood-pressure, to be explained by the paralysis of the smaller and a consequent emptying of the larger vessels. The vaso-motor centre maybe influenced that is, inhibited, or possibly strengthened by impulses coming from the periphery. Such impulses may be sent to the centre along any sensory nerve, but in 1866 Cyon and Ludwig discovered a nerve which apparently exercises this function to a remarkable extent. In the rabbit it originates by two roots from the superior laryn- geal and from the vagus. Stimulation of the distal end of this nerve pro duces no effect, but stimulation of the cephalic end causes at once a great fall of blood -pressure in the arterial system and a diminution in the frequency of the pulse. As this nerve, therefore, inhibits, restrains, or depresses the activity of the vaso-motor centre it has received the name of the "depressor nerve of Cyon and Ludwig." It appears to influence chiefly the vaso - motor arrangements of the abdomen and lower extremities. Thus, after section of the splanchnics, which control the vessels of the abdominal viscera, it is said that excitation of the depressor does not produce nearly the same diminution of pressure in the carotid vessels. By the influence of the depressor a balance is kept up between the central and the peripheric circulations. Imagine the heart to be pumping blood through the vessels. If from some cause the smaller vessels become constricted so as to offer greater resistance to the passage of the blood, the arterial pressure in the larger vessels is increased, and the heart has more work to do to overcome this resistance. When the resistance reached a certain amount the heart would be in danger of exhaustion in endeavouring to over come it. But by the depressor this danger is removed, as an influence may pass from the heart along the fibres of the depressor to the vaso-motor centre, the effect of which is to inhibit the activity of this centre, and thus allow the smaller vessels to dilate. When this occurs, either locally, as in the abdomiiial region, or generally, the result is a depletion of the larger vessels, a consequent fall of pressure in them, and therefore less resistance to the efforts of the heart. Thus it would appear that in the heart itself there is an arrangement by which, to a certain extent, it governs its own work, and there is an adjust ment between the activity of the heart and distribution of blood throughout the body (see fig. 11). The vaso-motor nerves causing contraction of vessels have been called " vaso-constrictors " ; but there are other nerve-fibres possess ing the property of causing a dilatation instead of a contraction. These have been called " vaso-dilators. " Excitation of the chorda- tympani nerve, for example, causes the vessels of the sub-maxillary gland to dilate (see vol. xvii. p. 672). Erection, as it occurs in the penis, has long been known to depend on dilatation of vessels and consequent increased afflux of blood. Stimulation of the nerves of the sacral plexus may cause erection. But how do such nerve- fibres act ? It cannot be that they directly cause relaxation of the muscular fibres in the walls of the vessels. These contain layers of involuntary muscular fibres in the transverse and longitudinal directions, and it is difficult to understand how any contraction of fibres In either of these directions could possibly cause dilatation of the vessel. Probably the effect is brought about by the action of some kind of inhibitory mechanism. Ganglia abound in the walls of the vessels. From these, fibres pass to and from the muscular elements of the vessel. Such ganglia or local reflex centres may be supposed to be under the influence of two sets of nerve-fibres : (1) accelerating or strengthening, corresponding to the accelerating fibres that influence the heart ; and (2) inhibitory, like the fibres of the vagus distributed to the heart, having the power of restraining the action of the local ganglia. According to this view, the fibres in the chorda which cause dilatation of the vessels of the sub -maxillary gland on stimulation are vaso- inhibitory nerves. Nerves Influence of Nerves on Glands. This has already been described and under NUTRITION (voL xvii. p. 672), but the facts may be here glands, briefly summarized. A secreting gland is supplied with three sets of nerve-fibres, vaso-constrictor, vaso-dilator or vaso-inhibitory, and secretory. The first two regulate the distribution of blood in the gland, whilst the third set directly affects the activity of the secreting cells. According to Heidenhain, in addition to the vascu lar nerves supplying a gland there are secretory and trophic nerves. "Stimulation of secretory fibres leads to an increased flow of water, stimulation of the trophic to an increased secretion of specific sub stances and to an increased production of protoplasm " (Gamgee). The vaso-constrictor fibres of a gland are derived from the sympa thetic, and the vaso-dilator and secretory from the cerebro-spinal system. Classification of Nerve - Centres. Although these are usually Nerve- classified anatomically, according to the organ in which they are centres situated, they may also be arranged according to their functions, as classi- follows : (1) receptive centres, to which influences arrive which fled, may excite sensations (in grey matter of brain), or some kind of activity not associated with consciousness (reflex centres of the cord and of the brain) ; (2) psychical centres, connected with sensa tion in the sense of conscious perception, emotion, volition, and intellectual acts (in the grey matter of the brain) ; (3) diseliarginij centres, whence emanate influences which, according to structures at the other ends of the nerves connected with them, may cause movements, secretions, or changes in the calibre of vessels (in brain and spinal cord) ; (4) inhibitory centres, which inhibit, restrain, or arrest the actions of other centres. B. Special Physiology of Central Organs. General Physiological Anatomy. The central organs of the nervous system consist of ganglia or of what is called a " cerebro- spinal axis." The anatomy of the latter is described under AXA- TOMY, and some account of the gangliated cords in invertebrates and of the rudimentary nervous systems of the lower forms oi vertebrates will be found under the articles CRUSTACEA, INSECTS, AMPHIBIA, BIRDS, ICHTHYOLOCTY, &c. But, as one of the most effective ways of obtaining an intelligent conception of the com plicated nervous system of man and of the higher animals is to trace its various forms in the scale of animal existence, and to observe the close correspondence between complexity of structure and complexity of function, a short introductory review of its comparative anatomy, from the physiological side, will here bo given. In the first place, we find that the different forms of nervous systems may be divided into (a) those consisting of ganglia or chains of ganglia, as found throughout the invertebrates, and (li) those having a great axis of nervous matter forming a brain and spinal cord, the cerebro-spinal axis, as seen in vertebrates. Comparative View of Nervous System of Invertebrates. In the Nervous simplest forms of animals the protoplasmic cell is the seat of system sensation and of motion ; but as the contractile or muscular of inver- layers become more marked sensation is relegated to the cells of tebrates. the ectoderm, or outer layer of the body. As portions of this sensory layer become of higher value to the organism, their protec tion is accomplished by some of the sensory cells sinking into the body of the organism so as to be covered by less important structures. The portions, originally of the surface, thus differen tiated and protected become ganglia, and processes pass from them on the one hand to cells in the periphery, so that they may still be influenced by external energies, and on the other to the contractile parts of the organism by which movements are accomplished. Still higher in the scale of life, the ganglia are connected by inter- nuncial fibres, and the plan of the primitive nervous system bears a relation to the general type of structure of the animal. Thus in radiate animals the gangliated cords show a radiated arrangement, and when the animal form is bilateral and symmetrical the nervous arrangements are on the same type. It is also to be noted that the ganglion specially connected with the rudimentary organs of sense, attains a size and importance proportionate to the development of the sense-organs. The nerves of the sense-organs are chiefly con nected with the supra-cesophageal ganglion, which thus may bo looked on as a rudimentary brain. When the body of the animal becomes more complicated by the development of similar segments (or metameres), we find that by a reduplication, as it were, of the subcesophageal ganglion a ventral chain of ganglia is formed, a pair of ganglia for each segment, the individual ganglia being connected by longitudinal commissures. Such an arrangement is seen in the ringed worms and in arthropods. The next step is a fusion of ganglia into masses, according to the size and importance of the part of the body to be innervated (see vol. vi. p. 636, figs. 7 and 9). No trace of a nervous system can be detected in Protozoa. The Scyphomodu- soid forms of Hydrozoa show nerve-fibres and ganglion-cells (Schiifer) in the sub-umbrella and around the tentaculo-cysts (see vol. xii. p. 552, tig. 15), and in the Hydromedusoid forms the nerve-ganglion cells form a ring round the margin of the disk. In some of the Actinmna (anemones, &c.) fusiform gan- glionic cells united by nerve-fibres are said to exist (P. M. Duncan). In all the worms (Vermes) the most important central organs of the nervous system are placed in the anterior part of the body near the beginning of the alimentary canal. If they have a distinct head the nervous organ is in it and supplies branches to the sense-organs. From thence nerve-trunks radiate to the peri phery of the body, often in the form of two longitudinal trunks on the ventral surface. Frequently there is a nervous ring round the oesophagus. Nerve- organs have been found in all the PlatyhelminOltl, Rotatoria, and Itrynzoa. The Xcmnthelminthes show a further advance. The central organ is placed on the
oesophagus, surrounding it as a ring, from which nerves radiate forwards and