CHAPTEE XIV. 



Fig. 280. 



DEVELOPMENT OF THE NERVOUS SYSTEM, ORGANS 

 OF SENSE, SKELETON, AND LIMBS. 



THE first trace of the cerebro-spinal axis in the embryo consists of the 

 two longitudinal folds of the external blastoderraic layer, which include 

 between them the median furrow, known as the " medullary groove" 

 (page 724). The two folds, after uniting by their corresponding edges 

 on the median line, over the back of the embryo, 

 convert the groove into a canal, the " medullary 

 canal ;" and it is within this canal that the cerebro- 

 spinal axis is formed. 



The mode of its formation is by the growth of 

 nervous matter upon the inner surface of the medul- 

 lary canal ; and this canal, which becomes the cerebro- 

 spinal canal, is accordingly lined with a secondary 

 internal sheath of nervous matter, which also has 

 the form of a tubular membranous canal, with a. con- 

 tinuous central cavity. This is the cerebro-spinal 

 axis, which thus forms a hollow cylindrical cord of 

 nervous matter, running in a longitudinal direction 

 within the cerebro-spinal canal. Anteriorly it ex- 

 pands into a bulbous enlargement corresponding to 

 the brain. Its middle portion, constituting the spinal 

 cord, is nearly cylindrical ; and posteriorly, at its 

 caudal extremity, it terminates by a pointed enlarge- 

 ment. 



The next change which shows itself is a division of the anterior bul- 

 bous enlargement into three secondary compartments or vesicles, par- 

 tially separated from each other by incomplete transverse constrictions. 

 These are known as the cerebral vesicles, from which the different parts 

 of the encephalon are afterward to be developed. The first or most 

 anterior vesicle is destined to form the hemispheres ; the second or 

 middle, the tubercula quadrigemina ; the third, or posterior, the medulla 

 oblongata. All three vesicles are still hollow, and their cavities com- 

 municate freely with each other through the intervening orifices. 



Very soon the anterior and posterior cerebral vesicles undergo a fur- 

 ther division, the middle one remaining undivided. The anterior vesicle 

 thus separates into two portions, of which the first, or larger, consti- 

 tutes the hemispheres, while the second, or smaller, becomes the optic 



(769) 



Formation of the 



CEREBRO-SriNAL 



Axis. a, b. Spinal 

 cord c. Cephalic ex- 

 tremity, d. Caudal 

 extremity. 



