13 



NERVOUS SYSTEM. 



NERVOUS SYSTEM. 



of the mantle and furnished with eight ganglia, from which filaments 

 proceed to the eight red spots which he supposes to be eyes ; whilst 

 the other is disposed around the entrance to the stomach, and is 

 furnished with four ganglia, from which filaments proceed to the 

 tentacula. A nervous ring has also been detected by Professor Agassiz 

 in Sarria, one of the Naked-Eyed Pulmogrades ; and he states that 

 it ia entirely composed of ganglionic cells. In Seriie, it is affirmed by 

 Dr. Grant that a nervous ring exists round the mouth, furnished with 

 eight ganglia, from each of which a filament passes towards the other 

 extremity of the body, while others are sent to the lips and tentacula. 

 In the Echinodermata however its manifestations are much less 

 equivocal. In the Asteriaa, for instance, a ring of nervous matter 

 surrounds the mouth, and sends three filaments to each of the rays ; 

 of these, one seems to traverse its length, while the two others are 

 distributed on the csecal prolongations of the stomach. In the species 

 examined and figured by Tiedemann, no ganglionic enlargements 

 of this ring appear to exist, and it seems not improbable that, as in 

 the Medusa, the entire ring is composed of vesicular nervous matter ; 

 but this element ia usually collected into distinct ganglia, which are 

 found at those points of the ring whence the branches diverge, the 

 number of the ganglia being always equal to that of the rays. In 

 those species which possess ocelli at the extremities of the rays, the 

 nervous chord proceeding towards each swells into a minute ganglion 

 in its neighbourhood. In the Echinus the arrangement of the nervous 

 system follows the same general plan ; the filaments which diverge 

 from the oral ring being distributed (in the absence of rays) to the 

 complicated dental apparatus, whilst others pass along the course of 

 the vessels to the digestive organs. The transition between the Radiata 

 and Articulata, presented by the Hololhuria and Sipunculiu, is pecu- 

 liarly well marked in the nervous system of these animals ; for the 

 ring which encircles the mouth is here comparatively small, but a 

 single or double non-gangliated filament traverses the length of their 

 prolonged bodies, running near the abdominal surface (which ia their 

 situation in the articulated classes) and giving-off transverse branches. 



When we compare the character of the nervous system of these 

 Radiated classes with that of the higher animals of more hetero- 

 geneous structure, we find that every segment of the body which is 

 similar to the rest ia connected with a ganglionic centre that seems 

 to be subservient to the functions of its own division alone, and to 

 have little communication with, or dependence upon, the remainder; 

 these centres being all apparently similar to each other in their 

 endowments. 



Among the Molluscous classes there is no radiate or longitudinal 

 multiplication of parts, the only repetition being on the two sides of 

 the median plane. It is chiefly in the organs of animal life that this 

 bi-lateral symmetry is observable, the symmetry of their nutritive 

 apparatus being obscured by the unequal development of its different 

 parts; and the predominance of the latter in their organisation 

 impresses itself (so to speak) upon their nervous system, which is not 

 formed until a late period of development, and which shows a want 

 of constancy in the relative position of its centres, that is, in striking 

 contrast with the uniformity of the plan which ia so obvious in the 

 nervous systems of Articulata and Vertebrata. Of these centres there 

 are typically three pairs : 1. The Cephalic Ganglia, which lie either 

 at the sides of the oesophagus, or above it ; and may be either dis- 

 joined, although connected by a commissure, or fused into one mass, 

 which ia usually bi-lobed; this gives off nervea to the labial and 

 olfactory tentacula, to the eyea, and to the muscular apparatus of the 

 mouth ; and upon either of these nerves accessory ganglia may bo 

 developed. 2. The Pedal Ganglia, which are commonly fused into one 

 mass which is situated below the oesophagus, and are connected with 

 the cephalic ganglia by a commissure! band on each side, forming a 

 ring which encirclea that canal ; though, in the Niulibrancliiala and 

 some other Gasteropoda, the pedal ganglion of each side is fused into 

 one mass with the corresponding cephalic ganglion : from the pedal 

 ganglion are given off nerves to the foot, and also to the organs of 

 hearing, when those are not actually lodged in them, as frequently 

 happens. 3. The Parieto-Splanchnic Ganglia, which are usually found 

 in the posterior part of the body, and are connected by commiasural 

 bands both with the cephalic and with the pedal ganglia : these give 

 off nervea to the muscular and sensitive parietes of the body, to the 

 shell-muscle or muscles, to the branchial apparatus, and to the heart 

 and large vessels. The function of these is divided, in the higher 

 Mollutca, between two or more pairs of ganglia; and the proper 

 visceral or sympathetic system becomes more distinct from it. 



[MOI.I.I V A.] 



The nervous system of the Molluica becomes gradually more com- 

 plicated as we ascend from the lower to the higher forms. It is in 

 the Cephalopoda, which stand at the head of the Molluscan series, that 

 we find an approach towards the atructure of the Vertebrate animals. 

 [CEPHALOPODA ; NAUTILID*.] 



The plan on which the nervoua system is distributed in the ub- 

 kiogdom Articulata exhibits a remarkable uniformity throughout the 

 whole series, whilst its character gradually becomes more elevated as 

 we trace it from the lowest to the highest divisions of the group. It 

 usually consists of a double nervous chord, studded with ganglia at 

 intervals ; and the more alike the different segments, the more equal 

 are these ganglia. The two filaments of the nervous chord are some- 



times at a considerable distance one from the other, and their ganglia 

 distinct ; but more frequently they are in close apposition, and the 

 ganglia appear single and common to both. That which may be 

 regarded as the typical conformation of the nervous system of this 

 group is seen in the gangliouic chord of Scolopendra, or in that of the 

 larva of moat insects, such as that of Sphinx Ligustri. Here we see 

 the nervous chord nearly uniform throughout, its two halves being 

 separated however at the anterior portion of the body ; the ganglia 

 are disposed at tolerably regular intervals, are similar to each other 

 in size (with the exception of the laat, which is formed by the coales- 

 cence of two), and every one supplies its own segment, having little 

 connection with any other. The two filaments of the chord diverge 

 behind the head to inclose the oesophagus ; above which we find a 

 pair of ganglia that receive the nerves of the eyes and antennae. We 

 shall find that, in the higher classes, the inequality in the formation 

 and office of the different segments, and the increased powers of special 

 sensation, involve a considerable change in the nervoua system, which 

 is concentrated about the head and thorax. In the amplest Vermiform 

 tribes, on the other hand, we lose all trace of separate ganglia, the 

 nervous chord passing without evident enlargement from one extremity 

 to the other. Whatever may be the degree of multiplication of the 

 ganglia- of the trunk, they seem but repetitions one of another ; the 

 functions of each segment being the same with those of the rest. The 

 cephalic ganglia however are always larger and more important ; they 

 are connected with the organs of special sense ; and they evidently 

 possess a power of directing and controlling the movements of the 

 entire body, whilst the power of each ganglion of the trunk is confined 

 to its own segment. The longitudinal gaugliated chord of Articulata 

 occupiea a position which seems at first sight altogether different from 

 that of the nervous system of Vertebrated animals, being found in the 

 neighbourhood of the ventral or inferior surface of their bodies, instead 

 of lying just beneath their dorsal or upper surface. From the history 

 of their development however, and from some other considerations, it 

 has been suggested that the whole body of these animals may be 

 considered as in an inverted position ; the part in which the segmen- 

 tation is first distinguished in insects being the real equivalent of the 

 dorsal region in Vertebrata, and that over which the germiual mem- 

 brane is the laat to close in being homologous with the ventral region. 

 This view applies also to the position of the ' dorsal vessel,' which 

 would then be on the ventral side of the axis, as in Vertebrata, 

 Regarded under this aspect, the longitudinal nervous tract of Articu- 

 lata corresponds with the spinal chord of Vertebrated animals in posi- 

 tion, as we shall find that it does in* function. 



When the structure of the chain of ganglia is more particularly 

 inquired into, it ia found to consist of two distinct tracts; one of 

 which ia composed of nerve-fibres only, and passes backwards from 

 the cephalic ganglia over the surface of all the ganglia of the trunk, 

 giving off branches to the nervea that proceed from them ; whilst the 

 other includes the ganglia themselves. Hence, as in the Mollusca, 

 every part of the body has two sets of nervous connections ; one with 

 the cephalic ganglia ; and the other with the ganglion of its own seg- 

 ment. Impressions made upon the afferent fibres, which proceed 

 from any part of the body to the cephalic ganglia, become sensations 

 when conveyed to the latter ; whilst, in correspondence to these, the 

 consensual impulses, operating through the cephalic ganglia, harinomae 

 and direct the general movements of the body, by means of the 

 efferent nervea proceeding from them. For the lower reflex opera- 

 tions, on the other hand, the ganglia of the ventral chord are sufficient; 

 each one ministering to the actions of its own segment, and, to a 

 certain extent also, to those of other segments. It has been ascertained 

 by the careful dissections of Mr. Newport, to whom we owe all our 

 most accurate knowledge of the structure of the nervous system in 

 articulated animals, that of the fibres constituting the roots by which 

 the nerves are implanted in the ganglia, some pass into the vesicular 

 matter of the ganglion, and, after coming into relation with the vesicu- 

 lar substance, pass out again on the same side ; whilst a second set, 

 after traveraing the vesicular matter, pass out by the trunks, proceed- 

 ing from the opposite side of the same ganglion ; whilst a third set 

 run along .the portion of the chord which connects the ganglia of 

 different segments, and enter the nervous trunks that issue from them, 

 at a distance of one or more ganglia above or below. Thus it appears, 

 that an impression conveyed by an afferent fibre to any ganglion, may 

 excite motion either in the muscles of the same aide of its own seg- 

 ment, or in those of the opposite aide, or in those of segments at a 

 greater or less distance, according to the point at which the efferent 

 fibres leave the chord. And as the function of these ganglia is altogether 

 related to the locomotive actions of the segments, we may regard 

 them as so many repititions of the pedal ganglia of the Mollusca ; 

 their multiplication being in precise accordance with that of the 

 instruments which they supply. [INSECTA ; CRUSTACEA.] 



Proceeding to the Vertebrated Seriea we find their Nervous System 

 constitutes a far more important portion of the entire organism than 

 it does in any Invertebrated animal ; and that, in ita most charac- 

 teristic forms, it combines the locomotive centres of the Articulata 

 with the sensorial centres of the Mottiuca, possessing in addition two 

 organs, the Cerebrum and Cerebellum, to which nothing distinctly 

 analogous can be detected in any of the inferior classes. That which 

 may be regarded as the fundamental portion of the nervous system in 



