258 DESIGN IN NATURE 



unstriped muscles of the body generally (heart,^ blood-vessels, &c.), and also to the glandular organs. Some viscera, 

 however, such as the heart, lungs, the upper and lower parts of the ahmentary canal, &c., in addition receive branches 

 direct from the cerebro-spinal system ; such as are not so suppUed being provided by nerves derived originally from 

 the cerebro-spinal nerves through their sympathetic plexuses. The unstriped muscles of the viscera and other parts 

 are also under the influence of fibres derived from the cerebro-spinal nerves. 



The account given of the nature and relations of the gangha and nerve fibres of the cerebro-spinal and 

 sympathetic systems is scarcely less conflicting than that given of the leading divisions of the great nerve centres 

 themselves. 



The sympathetic system structurally consists of meduUated fibres ; the fibres being remarkable for their com- 

 paratively very small size. They pass by means of the white rami communicantes from certain of the cerebro-spinal 

 nerves into the cord and ganglia of the sympathetic. The cerebro-spinal system is composed of white and grey 

 nerve substance in varying proportions. 



The trunks of the great sympathetic nerve, as explained, are two in number — one on either side of the vertebral 

 column. They begin at the base of the skull and terminate at the coccyx. The ganglia composing them are united 

 by nerve fibres from above downwards, from below upwards, and transversely. They are also united by nerve fda- 

 ments with the spinal nerves and their ganglia. Superiorly, they are connected with nerve plexuses which enter 

 the cranial cavity. 



It may be stated generally that the cerebro-spinal system of nerves, with its appropriate ganglia and nerve 

 fibres, presides over the striped voluntary muscles and movements of the body, and that the sympathetic system 

 of nerves, with its well-developed ganglia and nerve plexuses, presides over the unstriped involuntary muscles and 

 movements. This is true up to a point, for, as explained, the cerebro-spinal and sympathetic systems in 

 several cases interchange nerve fibres, and both systems supply nerves to the same structures. That the cerebro- 

 spinal and sympathetic systems of nerves do exercise a regulating power over the striped and unstriped muscles 

 in the higher animals seems proved by experiment, where the division and stimulation of certain nerves retard or 

 slow the movements of the organs or the parts to which they are distributed ; the division and stimulation of other 

 nerves accelerating or quickening the movements. The retarding or slowing nerves are known as inhibitory nerves, 

 those which accelerate and quicken as accelerator nerves. 



The famous Sir Charles Bell was the first strongly to direct attention to the existence of separate sensory and 

 motor nerves in animals, and relied for proof on the division and stimulation of the nerves themselves. In this 

 new departure in 1811^ he was followed by Majeridie ^ in 1822. Other original workers, such as the brothers 

 Weber, Claude Bernard, Brown-Sequard, and Dr. John Reid, soon enrolled themselves under the new banner. Dr. 

 John Reid, one of my predecessors in the Chandos Chair of Anatomy and Medicine in the University of St. 

 Andrews, especially distinguished himself in the new field by a series of papers at once thoughtful and brilhant, the 

 first of which saw the hght in 1835.* 



In dividing and stimulating the nerves in the Uving animal it is important to bear in mind that shock to the 

 system may largely vitiate the result, and render the experiments more or less futile. This is especially the case 

 where large nerves or branches of large nerves are divided or mutilated. The rule seems to be that the larger the 

 nerves experimented with, the less rehable the results. The shock caused by the experiment largely accounts for 

 what is here stated. Nor must certain fundamental facts be lost sight of. As already explained, plants and the 

 lowest animals are not provided with a nervous system, yet they perform all the important functions of life in its 

 absence. As, however, a nervous system in certain living forms is not required, it would be a mistake to attach 

 too much importance to it when present, and to experiments performed on it. 



From the brief account given of the nervous system as a whole, it will readily be inferred that it is no easy 

 matter to deal satisfactorily and conclusively with the reflex acts in animals. 



§52. Nerve Reflexes in Animals - Definitions of Reflex Acts ~ Subjects connected with Reflex 

 Manifestations, &c. 



By reflex acts in the higher animals are generally understood acts performed by a portion of the nervous system 

 (mamly the spinal cord), plus gangha, sensory, and motor nerves, &c., to the exclusion of the brain. The acts are 

 said to be involuntary and non-intellectual, and yet, in many cases, purposive. They have for their object the 



vohmteiT i.rascL^^*''°"^'' '"' '"''°'""*'"'y "'"'"'''• '^ •'"•"Po^^d of striated muscle. It forms a connecting link between the involuntary and 



I "Idea of a New Anatomy of the Nervous System," printed privately in the year mentioned (1811). 

 ^ Original experiment by Majendie published in the Jom: de Physiol, in 1822. 



"An Experimental Im'estigation into the Functions of tlie Eighth Pair of Nerves, or the Glosso-Pharyngeal, rneumo-trastric nnd SninRl 

 Accessory, lirst published m 18.35. For reprint see Reid's collected works, published thirteen years later, namely, in 1848. ' ^ 



