412 DESIGN IN NATURE 



The pre-arrangement and design so conspicuous in the development of the blood-corpuscles, blood-vessels, 

 muscles, and bones, is emphasised and accentuated in the case of the nervous system. This may be said to form 

 the keystone of the lofty and superlatively beautiful organic arch. The nervous system in man has a cellular 

 origin, and grows hke all other parts of the body. It becomes more and more complex as differentiation proceeds. 

 Structurally and functionally it attains to the most exalted position. Originally simple, it ultimately consists of 

 an extraordinary assemblage of nerve cells, ganglia, and nerve fibres. These, in one sense, are independent, in 

 another they are interdependent and inextricably interwoven. Collectively they form the organ of the mind. 

 The several portions of the brain are to be regarded as expansions of the spinal cord, as the several parts of the 

 skull are to be regarded as expansions of the vertebrae forming the vertebral column. The spinal cord and brain are 

 segmented, highly-symmetrical structures, and consist of pairs of gangha arranged on either side of the mesial hne 

 of the body at regular distances. The segmentation is indicated by the beaded outline of the medulla oblongata in 

 the embryo ; the medulla being the first expansion of the spinal column. The double chain of gangha referred 

 to are connected with each other longitudinally and transversely by commissural nerve-fibres. They are also con- 

 nected with symmetrical sets of sensory and motor nerves which extend to all parts of the body, and by the aid 

 of which, in the adult, they receive messages from the outer world through the skin and sense organs, and despatch 

 commands to the voluntary muscles which they set in motion. The following is the description of the origin of the 

 nervous system as given by Quain : ^ " The whole of the central nervous system takes origin from the thickened 

 walls of a dorsally situated axial groove, subsequently converted into a canal, which runs forwards in front of the 

 primitive streak, and the anterior end of which becomes enlarged and converted by constrictions into three suc- 

 cessive vesicles, around which the several parts of the brain are formed, and which are known as the primary 

 cerebral vesicles (Plate xci.. Kg. 1 ; Plate xcii.. Figs. 5 and 6). The remainder of the neural canal is of nearly 

 uniform diameter, and its walls become converted into the substance of the spinal cord, while the cavity itself 

 becomes eventually the central canal of the cord. The walls of the neural groove are composed of epiblast, and 

 it therefore follows that the whole structure of the central nervous system is laid down in epiblast, and consists 

 in the main of more or less modified epiblastic elements, except where mesoblastic tissues subsequently penetrate 

 into it, convejdng blood-vessels into its substance. As was shown by Balfour, the same is in all probabihty true 

 for all the nerves of the body, cranial and spinal. 



" The cord is at first oblong in section, with an angular depression in each side which serves to mark off the 

 situation of the future posterior columns and their corresponding grey matter from the antero-lateral region. These 

 two parts of the lateral neural epiblast may be distinguished as the dorso-lateral (alar) and the ventro-lateral (basal) 

 laminae ; with the former, the afferent nerve fibres become connected, whilst from the latter the efferent fibres take 

 origin (His). In the human embryo of six weeks, they are well marked off from one another, and their respective 

 connections with the posterior and anterior nerve roots are very distinct. The nerve fibres of the white columns 

 are at first entirely non-medullated, and the white substance has a greyish transparent appearance. The medullary 

 sheath is not formed simultaneously in all parts, but appears at different times in different parts corresponding 

 with the tracts of conduction : the last of these tracts to become meduUated are the pyramidal tracts. 



" The vesicles of the brain, which are at first three in number, become subdivided so as to form five in all, 

 which may be termed in succession from before back, the first, second, third, fourth, and fifth secondary vesicles. 

 Of these five parts the first two, wliich represent the cerebral and thalamic parts of the future brain (third ventricle), 

 are derived from the first primary vesicle, and the last two, the cerebellar and bulbar parts (fourth ventricle), 

 form the third primary vesicle, while the third, middle, or quadrigeminal part represents the undivided second 

 primary vesicle (Sylvian aqueduct). 



" The first and most striking change which occurs in the primary brain is the outgrowth on either side of the 

 first primary vesicle of a hollow protrusion {primary optic vesicle), which becomes developed eventually into optic 

 nerve and retina. 



" Subsequently another pair of hollow outgrowths sprout from the fore-brain, and these rapidly extend forwards, 

 laterally, and backwards ; they form the vesicles of the cerebral hemispheres. 



" The principal parts of the brain appear as thickenings in different parts of the walls of the vesicles. Thus 

 the corpora striata are formed in the floor of the hemisphere vesicles, whilst the principal mass of each hemisphere 

 is formed from the roof and sides (mantle) of those vesicles, and the olfactory lobes are hollow outgrowths from them. 

 The optic thalamus is formed by a thickening of the lateral wall of the second vesicle, the cavity of which comes 

 to be the main part of the third ventricle ; the corpora quadrigemina are thickenings in the roof, and the crura 

 cerebri thickenings of the sides and floor of the third vesicle, which becomes the aqueduct of Sylvius ; the cere- 

 bellum and pons are respectively thickenings of the roof and floor and the crura cerebelli of the sides of the fourth 



' Op. cit. pp. 57-63. 



