SYSTEM. PHYSIOLOGY 31 backwards. Often six strands of nerve run forwards, whilst a dorsal and a ventral trunk pass backwards. The size of these trunks depends on the length of the body. The cephalic ganglion is bilateral and is largely developed. In the Hirudinia and Annelida the cerebral ganglia are connected by commissures with a ventral cord, which, in turn, shows individual ganglia connected by commissures. Each ganglion consists of two equal portions with a transverse commissure, and in the higher forms they are so close as to form almost a single cord. It is also evident that the cerebral ganglia are composed of several ganglia fused together, and acquire functional importance as the sense-organs are more highly developed. In the Kchitiodermata the nervous system consists of a number of trunks placed ventrally and having a radial arrangement. Each of these trunks corresponds to the ventral ganglionic chain of the Annulata. In Asterida (star-fishes) each radial nerve consists of two bands thickened in the middle, and at the end there is a swelling connected with an optical appa ratus placed there. In the Echinus (sea-urchin) the nervous ring lies above the floor of the oral cavity, between the oesophagus and the lips of the ossicles of the masticatory apparatus. From this ring lateral branches issue which accom pany the branches of the ambulacra! vessels. In Holothuroida (sea-cucumbers) the nervous ring lies in front and near the mouth, and is thicker than the five nerves which it gives off, thus differing from the Asteroida and Kchinoida. The nervous system of the Arthropoda resembles that of the Annelida. There is a large ganglion above the oesophagus, the cerebral ganglion, united to a ventral ganglion by two commissures so as to form a nervous ring. From the ventral ganglion a series of ganglia united by commissures extends along the ventral surface of the body. The increased size of the cerebrum is the most striking characteristic, and no doubt bears a relation to the higher degree of development of the sense-organs, more especially those of sight. In some Crustacea the optic nerves arise from distinct lobes. As pointed out by Gegen bauer, when the optic organs are reduced or lost the cerebrum becomes so small as to be represented by nothing but a commissure. In the individuals having a large portion of the body composed of similar metameres the ganglia are regular in size, appearing in pairs. On the other hand, in the Thoracostraca (crabs, <fec.) the anterior ganglionic masses are fused into larger masses so as to correspond to the concrescence of the anterior metameres into a cephalo-thorax. In the abdominal portion of the body, where the metameres are small, distinct, and more or less regular, the ganglia are also distinct and in pairs. In the Protracheata (Peripattis) the nervous system is simpler, and consists of the oesophageal collar with a double ventral cord having no ganglia or swellings on it, although nerve-cells are distributed through it. In the Myriapoda there is a well-marked ventral cord, with ganglia corresponding to the metameres. In the Araclinida the ventral ganglia are often reduced in number and fused. They are characterized by the close connexion between the cerebral ganglia and the ventral cord, owing to the extreme shortness of the commissures (Gegenbauer). In the Scorpions the nervous system is richly segmented, and remarkable for the large size of the ganglion giving off the pedal nerves. The Spiders have a single large ganglion in the eephalo-thorax, no doubt consisting of several ganglia. In the Acarina (mites) the cerebral ganglion is extremely small, and the other ganglia are fused so as to form one single mass, giving off nerves all round. These minute animals show a remarkable degree of concentration of the nervous system. In Insecta (see fig. 12) the ventral cord traverses the whole length of the body, the ganglia being at equal distances, and all united by commissures. A This condition is well seen in the larval condition, and is like the permanent state of the Myriapoda. When the insect passes into the adult condition changes occur, con sisting essentially of the fusion o ganglia and a shortening of the com missures. The cerebral ganglion is composed primitively of three pairs, and in most cases does not unite Fio. 12. Typical forms of nervous system with the rest of the ventral cord, in invertebrates. A, in Serpida, a marine It shows hemispheres and a coin- annelid ; a, cephalic ganglion. B, in a crab; , cephalic ganglion; 6, ganglia fused under cephalo- thorax. C, in a plicated structure. The first gan glion of the ventral cord supplies the organs of the mouth ; the three white ant (Termes); a, cephalic ganglion, succeeding send nerves to the ap- (Gegenbauer.) pendages, feet, and wings ; the re maining ganglia are small, except the last, which supplies the generative organs. There is great variety among the Insecta in the number of ganglia in the ventral cord, but coalescence always indicates a higher type of structure. The nervous system of the Krachiopoda is formed of masses of ganglia near the oesophagus. From these nerve -fibres pass to various parts of the body. There is an o!sophageal ring, but the superior ganglion is very small, owing to the absence of higher sensory organs. In Molhisca (see vol. xvi. p. 635, fig. 1) the nervous system is divided into a superior ganglionic mass, which lies above the com mencement of the oesophagus the supra-oesophageal or cerebral ganglia and a ventral mass which is connected with the other by commissures, and forms the inferior or pedal ganglia. They are both paired. The cerebral ganglion is connected with the sense-organs. Both the cerebral and the pedal ganglionic masses really consist of ganglia fused together. This is well shown in some of the lower forms, in which the pedal ganglia are divided, and form an arrange ment like the ventral cord of the Annulata. The remarkable feature in the nervous system of Mollusca is the great development of the visceral ganglia and nerves supplying the heart, branchial apparatus, and generative organs (see vol. xvi. p. 643, figs. 17, 18 ; p. 644, figs. 20, 21, 22 ; p. 647, figs. 34, 35 ; p. 648, fig. 36). In the Lamellibraiichia the cerebral ganglia are very small, owing to the absence of a head and its sense-organs. In some forms they are placed so much to the side as to be united by a long commissure. There are also two pedal ganglia, of a size proportional to the degree of development of the foot. The visceral ganglionic mass is often the largest. It lies behind the posterior adductor muscle, and is united by long commissures to the cerebral ganglion (vol. xvi. p. 693, fig. 144). The nervous system of the Gastropoda is remark able for the large size of the cerebral ganglia. In the Pteropoda the cerebral ganglia either retain their lateral position or approach the pedal ganglia, with which the visceral ganglia are also fused. The three ganglionic masses, cerebral, pedal, and visceral, are also represented in the Cephalopoda, but they are more approximated by the shortening of the commissures. The ganglionic, masses consequently are of great size, and they are more differentiated than any other ganglia in invertebrates. It is possible to distinguish an outer grey layer, formed of ganglionic cells, surrounding a white layer, composed of fibres (vol. xvi. p. 670, figs. 113, 114, 115). Lastly, in Tunicata the nervous system is dorsal, instead of ventral, as in other invertebrates. It is developed from the ectoderm, or outermost layer of the embryo, by an infolding so as to form at first a groove and afterwards a tube. In the Ascidian larva; this nervous tube reaches throughout the length of the tail, and we have thus the remarkable condition of a dorsal-median nerve-cord, analogous to the cerebro-spinal system of vertebrates. Further, embryologists are of opinion that this rudimentary nervous system is the true central organ, although the greater portion of it Fig. 13. Fig. 14. disappears by the atrophy of the tail in the passage from the larval to the adult state. (Gegenbauer.) Comparative View of Nervous System of Vertebrates. To under- Nervous stand the structure of the complicated central nervous system of system of vertebrates, and to appreciate the physiological importance of its verte- various parts, it is necessary to trace its development in the embryo brates. and to note the various forms it presents from the lowest to the highest vertebrates. A consideration of the embryological and morphological aspects of the subject clears up many difficult problems which a study of the human nervous system, by far the most complicated physiological system in the body, fails to do, and in particular it gives an intelligent conception of its architecture, as seen both in simple and complex forms. The cerebro-spinal axis begins in the embryo as a tube of nervous matter produced by an infolding of the epiblast, or outermost embryonic layer. The tube widens at its anterior end, and, by constrictions in its wall, three primary cerebral vesicles are formed, which afterwards become the anterior, middle, and posterior parts of the brain. In the fully- developed condition the cavity of the tube remains as the central canal of the spinal cord and the ventricles of the brain, whilst the various parts of the brain and cord are formed by thickenings in its walls. , r/ ,^ j.,,. The three cerebral vesicles -J /( JM V- (<^ ^ Cerebral have been called the fore- ** "^ ) fl ) L vesicles brain, the mid-brain, and the hind-brain. A protrusion from the anterior cerebral vesicle, at first single, but afterwards divided by a median cleft, becomes the rudiment of the cerebral hemispheres (prosen- ccphala), the cavity remain ing in the adult condition as the lateral ventricle on each side. From each cerebral vesicle another hollow process protrudes which constitutes the olfactory lobe (rhinen- cephalon). What remains of the cavity of the first vesicle becomes the third ventricle (thalamenccphalon). In the ^ I outer and under walls of the proscnccphala a thickening is formed which becomes the corpora striata, two large bodies in the floor of the lateral ventricles of the adult brain, whilst the roof is modi- Fl - 14.-Embryo of dog, more advanced, c , . , ,, , , c , , seen from above (after Bischofl). The fu>,1 into the subsr,.inrfi of the lnedullarv canal is v no w close(l in ; Cj an . terior encephalic vesicle ; o, primitive optic vesicle ; cm, primitive auditory vesicle, opposite third encephalic vesicle; a??i, cephalic fold of amnion ; or, vitellinc veins entering heart posteriorly ; pr, proto-vertebral somites. (Quain s Ana tomy.) lami, a thin layer between the two constituting the taenia scmi- drcularis, and the Y-shaped canal passing from the cavity be tween the thalami to the cavities in the cerebral hemispheres (lateral ventricles) is the foramen of Monro. The floor of the third ventricle is produced into a conical process, the infundi- bulum, at the blind end of which is the pituitary body, or hypo- plnjsis ccrebri. The roof of this ventricle is very thin, and in con nexion with it is developed the pineal gland, or cpiphysis ccrebri. Transverse fibres pass from the one corpus striatum to the others, constituting the white, commissure, whilst the two optic thalami are connected by two grey commissures. In mammals the two cerebral hemispheres are connected by a large and important set of com- missnral fibres, forming the corpus callosum. In addition there are certain sets of longitudinal commissural fibres. Thus two sets of fibres arise from the floor of the third ventricle, arch upwards, and form the anterior pillars of the fornix. These are continued over the roof of the third ventricle and run backwards, constituting the body of the fornix. Behind this the bands diverge so as to form the posterior pillars of the fornix. In the higher vertebrates the upper lip of the foramen of Monro thickens, and becomes converted into a bundle of longitudinal fibres, which is continuous anteriorly with the anterior pillars of the fornix. These are continued back between the inner boundary of the cerebral hemisphere and the margin of the corpora striata and optic thalami, and project into the lateral ventricle, forming the hippocampus major. As in highlv- formed brains the corpus callosum passes across considerably above the level of the fornix, a portion of the inner wall of the hemisphere on each side and a space between are intercepted. The two inner walls constitute the sc2)tum lucidum, and the space the cavity of 13. Outline from above of embryo chick in first half of the second day. 1 to 2, three primary encephalic vesicles enclosed in front and at the sides by the cephalic fold ; 3, hinder extremity of medullary canal dilated into a rhomboid space in which is the primitive trace ; 4, 4, seven proto-vertebral somites. (Quain s Anatomy.) fied into the substance of the cerebral hemispheres. Im mediately behind the corpora striata, and in the floor of the thalamencephalon, two similar thickenings occur
which become the optic tha-