CEREBRO-SPINAL NERVOUS CENTRE. 695 



spinal nervous centre ; the lower forminc!^ the walls of the body whicli 

 enclose the great nutritive viscera in the thoracic, abdominal and pelvic 

 divisions of the trunk ; — along with wliich may be associated the parts 

 which form the face and jaws, and Avhich enclose the cavities of the 

 nose, month, and pharynx, incUiding in their substance the hyoid bone 

 and its accompanying branchial arches. 



The Cerebro-spinal nervous Centre. — The brain and spinal cord 

 have at first together the form of an elongated tube, of which the 

 primary wall is of nearly equal thickness throughout. The cjdindrical 

 portion in the region of the protovertebraa forms the spinal cord. In 

 the dilated cephalic portion, constituting the rudimentary brain, there 

 is from a very early period a partial division into three portions by 

 slight intervening constrictions of the wall of the medullary tube. 

 These constitute the three jn-imarij enrpphalic resides, and give rise in 

 the next stage of development to the five fundamental portions of the 

 brain usually recognised by embryologists and comparative anatomists, 

 viz., forebrain, interbrain, midbrain, hindbrain and aftcrbrain. The 

 general cavity enclosed by the inflection and union of the medullary 

 plates constitutes the mesial ventricles of the brain and the canal of the 

 spinal cord. 



Fig. 506. — Magnified side view of the Head Fig. 506. 



AND Upper Part op the Body op an 

 EnDRro-CHicK OF THE FouRTH Day (adapted 

 from Remak and Huxley). 



1, cliorda dorsalis ; 2, tlirce of the upper 

 priDiitive cervical vertebne ; C\ one of the vesicles 

 of the prosencephalon, with the nasal fossa be- 

 low ; C-, vesicle of the thalameacephalon, with 

 thg eye below it ; C'*, the middle cerebi-al vesicle ; 



C'*, the cerebellum, between which and the cer- C^~[- ^""^^ ^ ] JL'^>C<$J^^!-— ' TI 

 vical vertebra is the medulla oblongata. At the 

 anterior extremity of the chorda dorsalis, where 

 it reaches the ijost-spheuoid, is seen the rect- 

 angular bend of the middle of the cranium, 

 which takes i^lace at the sella turcica ; and in 

 front of tliis, towards the eye, the pointed infun- 

 dibulum ; V, the rudiment of the trigeminus 

 nerve ; VII, the facial ; VIII, the vagus ; IX, the 



hypoglossal ; in front and below these numbers respectively, first, the upper and lower 

 jaw, with the first cleft, which becomes the meatus auditorius externus ; and lower down 

 the second, third, and fourth arches and clefts in succession ; in front of these the aortic 

 bulb attaches the heart ; between VII aud VIII, the auditory vesicle. 



The 23"erves. — The peripheral nerves are formed, quite independently 

 of the nerve centres, in mesoblastic elements along with the vascular 

 and other tissue composing the parts in which they are distributed. 

 The anterior aud posterior roots of the spinal nerves and the roots of 

 the cranial nerves (excepting the optic, which has a special connection 

 with the brain) probably arise as outgrowths from the medullary wall 

 of the cerebro-spinal centres. 



Organs of the Senses. — To the earliest period also belongs the 

 formation of the rudiments of the principal organs of the senses, viz., 

 the eye, ear and nose. The mode of origin differs, however, in the 

 three. In the eye, which is the earliest to appear, the retina, or 

 nervous part, is an extension from the anterior encephalic vesicle, while 

 the lens is derived by development from an involuted portion of the 



