132 DESIGN IN NATURE 



chain. An impression received by the integument of any part of the body may be transmitted inward to its own 

 ganghon and thence reflected immediately outward, so as to produce a movement of the limbs belonging to that 

 articulation alone ; or it may be propagated, through the longitudinal commissures, forward or backward, and 

 produce simultaneous movements in several neighbouring articulations ; or, fmally, it may be propagated qmte 

 up to the anterior pair of gangha or ' brain,' where its reception will be accompanied with consciousness, and a 

 voluntary movement reflected back upon any or all of the limbs at once. The organs of special sense, also, com- 

 municate directly with the cerebral gangUa ; and impressions conveyed through them may accordingly give rise 

 to movements in any distant part of the body." i Similar reasoning, I would point out, is apphed to the nervous 

 system of man, especially to the gangha or nerve centres of the spinal cord. It is also applied to the nerve centres 

 of the head and other parts in their relation to the sense organs. Figures illustrating the comparative anatomy of 

 the nervous system of the lower and higher animals are given at Plate Ivii. which follows. 



The starfish is capable of controlhng its movements. It can move towards an object which it fancies, or away 

 from an object which it disHkes or fears. The element of voluntariness enters into all its acts. It takes full advantage 

 of the lines of communication which connect it with the outer world, and the hues of force which enable it to move 

 its several parts as a whole (Plate Ivii., Fig. 1, A). 



PLATE LVII 



This plate shows the beginnings and gradual advance of the nervous system as represented by ganglia and 

 sensory and motor nerves in the lower animals : also the progressive development of the spinal cord and brain 

 in the higher animals. The brain is to be regarded as an expansion of the spinal cord, and consists of essentially 

 the same nerve elements. 



Fig. 1. — Illustrates the nei've arrangements in the five-rayed brainless starfish. It consists of a circular commissural nerve ring 

 with five nerve centres or ganglia (one at the foot of each limb). Each ganglion receives and gives off sensory and motor nerves. 



A. a, Commissural nerve ring surrounding the mouth of the starfish ; h, oral aperture of starfish ; c, one of the five triangular- 

 shaped giinglionic masses ; d, motor nerves ; e, sensory nerves. The starfish in virtue of its nerve arrangements can receive sensory 

 impressions from without and send out motor impulses from within. The animal can feel and move voluntarily, and is in no sense 

 dependent for its movements on inherent irritability, stimulation, or so-called reflex action. The five-rayed starfish and its nervous 

 system are symmetrical (after Dalton). 



B. The Aplysia — an unsymmetrical mollusc — with an unsymmetrical nervous system. In tlie aplysia a rudimentary brain makes 

 its appearance. The cephalic or brain ganglion is composed of two small ganglionic masses fused together and connected with the 

 ganglionic nerve centres in all the other parts of the body by means of commissural nerve fibres. The ganglia receive and give off 

 sensory and motor nerve fibres, and the animal can, like the starfish, feel and move to given ends, irrespective of inherent irritability, 

 artificial stimulation, and so-called reflex action, a, Digestive or oesophageal ganglia ; b, cephalic or cerebral ganglia ; c c, pedal or 

 locomotory ganglia ; d, respiratory ganglion. The nervous system of the aplysia is symmetrical in the upper, and asymmetrical in 

 the lower part of the body. 



Fig. 2.— Nerve cells from a lumbar sympathetic ganglion without a sheath (A), and with a sheath (B), from the nervous system of an 

 adult man. The cell substance contains pigment of a vivid yellow tint, and is consequently darkly granular (after Max Schultze). 



Fig. 3.— a. Illustrates the nervous -system of the centipede (Scolopendra). It consists of a double chain of ganglia connected 

 together by longitudinal and transverse nerve commissures ; two ganglia with sensory and motor nerves being provided for each 

 articulation of the animal. The cephalic ganglia are increased in size and united to form a fairly well developed brain. The brain of 

 the centipede sends nerves to the antenna; and to the organs of special sense, a, Brain composed of two symmetrical portions united 

 longitudinally and transversely by nerve commissures ; c c', the longitudinal commissural fibres of the left side of the nervous system. 



B. Illustrates the nervous system of man (Homo napiens). The human nervous system is usually divided into a cerebro-spinal 

 and sympathetic portion— the former only being here represented. The cerebro-spinal svstem closely resembles that of the centipede 

 which furnishes its type. It consists of two symmetrical halves with collections of ganglia corresponding to the segments of the body 

 The gangha are provided with sensory and motor nerves, and are united longitudinally and transversely by nerve commissures to 

 secure harmonious working. 'Ihe peculiarity of the cerebro-spinal system in man is the enormous expansion of the cerebro-spinal 

 obes or hemispheres which grow upwards, forwards, and backwards, and so cover in the great ganglionic nerve centres situated at the 

 base of the brain a, Cerebral lobes or great brain (cerebrum) ; b, cerebellum ; c, cervical portion of spinal cord : c', dorsal portion 

 of spinal cord : d, brachial plexus and nerves going to the arms ; e, lumbar plexus and nerves going to the legs. 



Fig. 4.— Transverse sections of cervical portion of human spinal cord, showing the grey ganglionic nerve centres with their sensory 

 and motor nerves ihe grey ganglionic matter is arranged in two symmetrical crescentic masses within the cord : this being united 

 longitudinally and transversely by nervous commissures. The sensory and motor nerves are connected with the free extremities 

 (horns) of ihe crescentic grey substance and are white in colour. The cord is divided into two portions by anterior and posterior 

 fissures. It is further divided by the grey crescentic matter of the cord and the ingoing and outcoming sensory and motor nerves, so 

 that it may be said to consist of six longitudinal columns. « o o j i 



Numerals are employed in describing the figures given of tlie spinal cord, and the same numerals indicate the same parts. 



A. Anterior view of spinal cord. C. Upper surface of cord. 



B. View of right side of cord. D. The nerve roots and ganglia seen from below. 



(1), Ante rior median fissure of c ord ; (2), posterior median fissure of cord; (3), antero-lateral depression; (4), posterior lateral 

 1 "A Treatise on Human Physiology," by John C. Dalton, M.D., &c., 5th edition, piJ. 38S-388. 



