360 RESPIRATORY NERVES OF ARTICULATA. ganglia and their nerves are best seen in the front of the body, where the cords that pass between the ganglia diverge or separate from each other. This is shown on a larger scale in fig. 188; where A B, A B, are two pairs of ganglia in the thoracic region, connected by two cords which diverge from one another; and between these are seen the small respiratory ganglion a, and its branches bb. These branches are distributed to the air-tubes and other parts of the respiratory apparatus, and communicate with those of the other system. We shall find that, even in the highest Vertebrata, there is a portion of the nervous-centres which is set apart for the maintenance of the respiratory actions, and which may be regarded as the respiratory ganglion; though it is so closely connected with other parts of the mass as to seem but a part of it (§ 450). Fig.188.-PORTION OF nerves. 447. In the higher Invertebrata, among both the Articulated and the Molluscous classes, we find a tendency to the concentration of the ganglia into one or two masses,-carrying to a Fig. 189.-NERVOUS SYSTEM OF CRAB (Maia). ca, upper part of the shell laid open; a, antennæ; y, eyes; e, stomach; c, cephalic ganglion; no, optic nerves; co, oesophageal collar; ns, stomato-gastric nerves; , thoracic ganglionic mass; np, nerves of the legs, na, abdominal nerve. CONCENTRATION OF GANGLIA IN HIGHEST INVERTEBRATA. 361 per greater extent that which has been already noticed in the fect Insect (§ 441). Thus in the Spider, the cephalo-thorax contains a single large ganglion (t, fig. 46), from which all the legs are supplied. The same is the case in the Crab, whose nervous system is represented in fig. 189. Besides this mass, t, however, which is situated beneath the alimentary canal, there is a single or double cephalic ganglion, c, which receives the nerves from the organs of sense, and sends backwards, to communicate with the mass t, a pair of cords that separate to give passage to the oesophagus, round which they form a sort of collar co. And there are other small ganglia and nerves, connected with the operations of mastication and digestion, which are called stomato-gastric (from two Greek words, meaning the mouth and the stomach). 448. A similar concentration, though with a different arrangement of parts, is seen in the nervous system of the Poulp, one of the Cephalopoda (§ 111). There is still a nervous collar through which the oesophagus passes (a, fig. 190); but the organs of locomotion being the enlarged tentacula that surround the mouth, the nerves given off to them arise from ganglia that form part of the cephalic mass, b, b, instead of being located at a distance from it. At o are seen the optic nerves, proceeding from distinct ganglia; and at c is a heart-shaped ganglionic mass, which seems to bear more resemblance to the proper brain of higher animals, than does any that we elsewhere find in the Invertebrata. In front of this are two ganglia on the middle line, both of which belong to the stomato-gastric system, one supplying the lips and the other the pharynx. From the mass g, situated beneath the œsophagus, there pass backwards two cords m m, each of which has a ganglion e upon its course, and from this are given off nerves to the general surface of the mantle; and also other two cords, which run backwards to supply the viscera, and especially the gills,-each passing through a long narrow ganglion r, before entering them. It would seem as if the ganglia e and r corresponded with the ganglia c and в in the Aplysia; but as if, in consequence of the great enlargement of the cephalic mass, they were proportionally reduced in size. 449. In the nervous system of Vertebrated animals, the ganglia are no longer scattered through the body, but are 362 NERVOUS SYSTEM OF VERTEBRATA. united into one continuous mass; and this mass, constituting the Brain and Spinal Cord, is inclosed within the bony m Fig. 190.-NERVOUS SYSTEM OF OCTOPUS (POULP). casing formed by the skull and vertebral column, in such a manner as to be protected by it from injuries to which it would otherwise be continually liable (§§ 72, 73). We have seen that among the Invertebrated classes the nervous system has no such peculiar defence, but lies among the other sharing with them the protection afforded by the general hard envelope of the body. But in the Vertebrata, its development organs, NERVOUS SYSTEM OF VERTEBRATA. 363 is so much higher, and its importance so much greater, that special care is taken to guard it from injury.-The term brain is commonly applied to the whole mass of nervous matter contained within the cavity of the skull; but this consists of several distinct parts, which have obviously different characters. The principal mass in Man and the higher Vertebrata is that which is termed the Cerebrum (fig. 195, a); this occupies all the front and upper part of the cavity of the skull, and is divided into two halves or hemispheres by a membranous partition which passes from back to front along the middle line. Beneath this, at the back part of the skull, is another mass, b, much smaller, but still of considerable size, termed the Cerebellum; and this also is divided into two hemispheres. At the base or under side of the cerebrum, and completely covered-in by it, are two pairs of ganglia (1 and g, fig. 196), which belong to the nerves of smell and sight. We shall presently find that these are, relatively speaking, much larger in the lower Vertebrata than in the higher. 450. The several masses of nervous matter contained in the skull, are connected with each other and with the spinal cord by bands of nerve-fibres and tracts of vesicular substance, which serve to bring the brain into connexion with the nervetrunks issuing from the spinal cord. But the Spinal Cord has also distinct properties of its own, analogous to those which have been shown to exist in the chain of ganglia in Insects. The upper part of it, which passes-up into the cavity of the skull, is termed the Medulla Oblongata (f', fig. 197). This is connected with the nerves of respiration, mastication, and deglutition; and may be regarded as combining together the respiratory and the stomato-gastric systems of Invertebrata. The remainder of the spinal cord, which descends through the vertebral column, sends its nerves to the limbs and trunk; and may be regarded as analogous to the chain of ganglia by which the corresponding parts are supplied in Insects. 451. The nerves which issue from the Spinal Cord, all possess two sets of roots; one from the anterior portion of the cord, the other from its posterior portion (fig. 191). The fibres which come-off by these two sets of roots, soon unite into the trunk of the nerve, which thus possesses the properties common to both. It was the great discovery of Sir 364 NERVOUS SYSTEM OF VERTEBRATA. Charles Bell, that the posterior set of roots consists of those fibres that bring impressions from the body in general to the Spinal Cord; which impressions, if carried-on to the Brain, a d f become sensations. On the other hand, the anterior roots consist of fibres which convey motor influence from the Spinal Cord and Brain, to the muscles of the body. Thus if the spinal cord of an animal be laid bare, and the posterior set of roots be touched, acute pain is obviously produced; whilst, if the anterior roots be irritated, violent motions of the muscles supplied by that nerve are occasioned. Both these roots contain fibres that connect them with the brain as well as with the spinal cord; so that, through the same trunk, either of these centres may act upon the part. We shall presently find that there is good reason to believe the Brain to be the seat of sensibility and of voluntary power; whilst the Spinal Cord is the instrument of those reflex actions which take place automatically, as it were, without direction on the part of the animal, and which are concerned in the maintenance of the organic functions of the body, and in its preservation from injury. Fig. 191.-PORTION OF THE SPINAL CORD, Showing the origin of the nerves: a, spinal cord; b,posterior root; c, ganglion upon its course; d, anterior root; e, trunk formed by the union of both; f, branch. 452. The relative proportions which these different parts present, are very different in the several classes of Vertebrata. We find that among the lower, the Sensory Ganglia, or ganglionic centres immediately connected with the organs of sense (which are analogous to the cephalic ganglia of the Invertebrata), are very large, and occupy a considerable part of the cavity of the skull; whilst the Cerebrum and Cerebellum are comparatively small. The Cerebrum increases, as we ascend the scale, in proportion to the development of the intelligence, and the predominance which it gradually acquires over blind undesigning instinct (Chap. XIV.). Its greatest development is seen in Man.-The Cerebellum seems to be connected with muscular motion, and to bear a proportion in size with the variety and complexity of the movements which the animal performs, serving to harmonise these and blend them together (§ 480). On the other hand, the Spinal Cord, and the nerves |