184 ANIMAL PHYSIOLOGY. 



pressure, because the substance which binds their nervous 

 elements is nearly as delicate as those elements themselves. 

 The nerves emerging from the brain and spinal cord are 

 likewise delicate before piercing the fibrous membrane (dura 

 mater) which surrounds those structures and mingles its fibres 

 with the nerve trunks as they issue from it. 



138. Cerebro-Spinal Axis. The brain and spinal cord 

 form one continuous structure, the cerebro-spinal axis; and 

 this, in early embryonic life, as it may be studied, for 

 example, in a hen's egg which has been hatched for two or 

 three days, lies originally in an open groove on the sur- 

 face of the body (fig. 149). But soon the edges of the 

 groove come together, converting it into a tube, which is the 

 future cranial cavity and spinal canal; and in like manner 

 the edges of the structure within the canal unite, and the 

 cerebro-spinal axis thus becomes tubular also (fig. 153). But 

 the part which forms the brain is very unequally developed, 

 being bent on itself, its cavity swollen at some parts, con- 

 stricted at others, and its walls thin or absent at one place 

 and thick at another; while the spinal cord retains its cylin- 

 drical shape, although its walls are thickened, and its hollow, 

 which remains as the central canal, is so small that it cannot 

 be seen in the human subject without the aid of a micro- 

 scope. 



Two descriptions of substance are distinguished in the 

 cerebro-spinal axis, the white and the grey or cineritious. 

 The white matter consists of nerves without corpuscles, and 

 owes its whiteness to the medullary sheaths of the nerves; 

 while the grey matter has only very minute medullated 

 fibres, and contains in different parts a great variety of 

 dispositions of nerve - corpuscles. Notwithstanding these 

 varieties of the grey matter, and that the difference in 

 colour between it and the white depends on the greater 

 vascularity of the grey, and on the smaller proportion borne 

 by the medullary sheaths to the other textural elements; 

 yet, as tracts of grey matter without corpuscles are very 

 small, and the white matter has no corpuscles anywhere, the 

 grey matter may be considered as ganglionic, and the white 

 as merely conducting. Along the whole length of the 

 original cerebro-spinal cylinder, grey matter lies next the 



