398 BIOLOGICAL PHYSICS 



second and first pairs of cephalic nerves, being exceptions 

 from this manner of meningeal accompaniment, require a 

 few words of special reference. Instead of beginning their 

 course of development directly from the cerebral periphery, 

 they are projected as proper cerebral textures, or processes, 

 for a considerable distance along the base of the brain, 

 before the nerves proper, to which these textures, or 

 processes, lead, become evolved, and organised. In the 

 early neuro-vesicular stage of embryonic growth, these two 

 pairs of special sense nerves originate, as projections, or 

 swellings, of the nascent fore-brain, and surround them- 

 selves with continuations of the nascent meninges, as they 



FIG. 129. LONGITUDINAL SECTION THROUGH THE HEAD OF AN EMBRYO 

 OF FOUR WEEKS. *-. (From Kolliker.) 



z, anterior encephalic vesicle, cerebral portion ; z, interbrain ; m, midbrain ; h, cere- 

 bellum ; n, medulla pblongata ; no and a, optic vesicle ; 0, auditory depression ; 

 /, centre of basi-cranial flexure ; f, lateral and hinder parts of tentorium ; /, the 

 fold of epiblast which forms the hypophysis cerebri. 



leave the developing cranial cavity, and so dispose of these 

 meningeal textures that they become part and parcel of the 

 proper sense organs ; almost the same may be said of the 

 auditory division of the fifth pair. 



The vital process of developmental adaptation of these 

 meningeal textures to the requirements of sense organs, is 

 one of the transcendental examples of nature's methods of 

 turning the common into the uncommon, and of utilising 

 the ordinary, and immediately available, formative materials, 

 for the accomplishment, or production, of extraordinary 

 organic purposes and ends. (On this subject we claim that 

 another Briclgewater Treatise might be witten, and occasion 

 for literary inspiration, and graphic delineation, amply pro- 

 vided for, in their highest flights.) 



The optic vesicular (Fig. 129) enlargements of the 



