IOO 



BIOLOGICAL PHYSICS 



cerebro-spinal fluid. This fluid, as are all fluids within 

 an organised body, is not allowed to flow at large, but 

 is confined within well-defined limits, secured here by 

 the growth of certain containing membranes, which ulti- 

 mately become the meninges of the brain and spinal 

 cord, and the neurilemmar coverings of the nerves 

 respectively. 



The growth of these containing membranes, and the 

 circulation and retention of their contained fluid, necessi- 



FIG. 30. VERTICAL SECTION OF THE HEAD IN EARLY EMBRYOES OF THE 

 RABBIT. Magnified. (From Mihalkovics.) 



A. From an embryo of five millimetres long. 



B. From an embryo of six millimetres long. 



In A, the faucial opening is still closed ; in B, it is formed ; c, anterior cerebral 

 vesicle; me, meso-cerebrum ; mo, medulla oblongata; tit, medullary layer ; if, 

 infundibulum ; am, amnion ; spe, spheno-ethmoidal, be, central (dorsum sellae), 

 and spa, spheno-occipital parts of the basis cranii ; h, heart \f, anterior extremity 

 of primitive alimentary canal and opening (later) of the fauces ; /, cephalic portion 

 of primitive intestine ; ch, notochord ; py, buccal and pituitary involution. 



tates the provision of a series of excretory mechanisms, 

 or organisms, whereby its quantity can be maintained 

 in " balanced ratio," and the ceaselessly recurring dis- 

 turbances and vicissitudes of the central nervous system 

 neutralised. Here, then, in this connection, we may see 

 in the very early appearance of the pituitary and pineal 

 mechanisms, or organisms, examples of that provision, as 

 well as a vindication of the views of the earliest anatomists 

 regarding their truly glandular structure. The embryonic 

 evolution of the pituitary gland (Figs. 28, 30, 32) in 

 structure and function is marvellously completed and 



