758 



THE NERVOUS SYSTEM 



thicken uniformly and, to give rise to the form of the adult brain, the anterior and the posterior 

 of the three vesicles give off secondary vesicles. 



The walls of the posterior primary vesicle give rise to the posterior of the main divisions 

 of the brain, the hind brain or rhomhencephalon, the cerebellum developing from the anterior 

 portion only of its dorsal wall, and the medulla oblongata and pons from its ventral wall. Its 

 cavity persists and enlarges into the fourth ventricle of the adult, while the posterior portion 

 of its dorsal wall does not develop functional nervous tissue at all but persists as a thin membrane 

 known as the chorioid tela of the fourth ventricle. The cells which form the ganglia of the audi- 

 tory and vestibular nerves arise from the dorsolateral regions of this vesicle. 5 



From the middle primary vesicle comes the mid-brain or mesencephalon, the corprora quad- j 

 rigemina [colliculi] developing from its entire dorsal wall and the cerebral peduncles occupying 

 its ventral wall. The constriction between the middle and posterior vesicles becomes the 

 isthmus of the rhombencephalon. 



The anterior or first primary vesicle undergoes greater elaboration than either of the other 

 two. At an early period it gives off a series of secondary vesicles or diverticula. First, two 

 ventrolateral outpouchings occur, the optic vesicles, which later become the optic stalks and 

 optic cups of the embryo. A medial protuberance becomes evident in its antero-dorsal wall 

 and from each side of this quickly starts a lateral diverticulum. The two lateral diverticula 

 thus arising from the protuberance are the beginning of the two cerebral hemispheres or the 

 telencephalon, and the vesicular cavities contained persist as the two lateral ventricles of the 

 brain. Soon, each of these vesicular rudiments of the hemispheres gives off ventrally from its 

 anterior part a narrow tube-like diverticulum, each continuous into the parent primary vesicle. 

 These are the olfactory vesicles which are transformed into the olfactory bulbs and olfactory 

 tracts of the adult encephalon. (See fig. 598, B. and C.) As development proceeds, the 

 cavities of the olfactory vesicles become occluded in man. However, in many of those animals 



Fig. 601. — Diagram of Mesial Section of the Human Brain showing the Segments and 

 THE Flexures and the Expansion of the Cerebral Hemispheres over the Other 

 Portions of the Brain. The Thalamus is not shown. 



Epiphysis 



Corpora quadrigemina 

 (.mid-brain) 



Cerebral hemisphere 

 Corpus callosum 



Septum pellucidum 



fej^ Fornix 



Foramen of Monro 



Third ventricle 



Fourth ventricle 



Hypophysis 

 Cerebral peduncle 



Pons 



Medulla oblongata 



Spinal cord 



(hind -brain) 



I 



in which the olfactory apparatus attains greater relative development than in man, these cavi- 

 ties persist as the olfactory ventricles. The cavities of the optic vesicles never persist as ven- 

 tricles in the adult. They form stalks which represent the future courses of the optic nerves, 

 while from their extremities are developed the retinae, portions of the ciliary bodies and portions 

 of the iris of the ocular bulbs. 



In addition to that which forms the cerebral hemispheres, the remaining portion of the 

 anterior j)rimary vesi(;le becomes the diencephalon or inter-brain. The lateral walls of this part 

 thicken to form the thalami, the posterior end of its dorsal wall gives off a secondary vesicle 

 which becomes the })ineal body or epiphysis, and from its ventral wall projects the infundibular 

 recess which becomes the posterior lobe of the hypophysis with its infundibulum and tuber 

 cinereum. 



The adult human brain is characterised by the preponderant develoi)ment of the cerebral 

 hemispheres. The secondary vesicles forming these expand till, held within the cranial cavity, 

 the hemispheres come to extend posteriorly completely over the thalamencephalon and the 

 me.sencep]ialoii and even overlap the («rebellum to its posterior border. Their cavities, which 

 persist from tlicir origin from the anterior primary vesicle, are correspondingly large (the lateral 

 ventricles) and compri.se two of thcs four rcnlrirlcs of the adult brain. The third ventricle be- 

 comes a narrow cavity situated between the two thalami. It represents the original cavity 

 of the anterior primary v(!sicle from which the stru(^tvn-es above mentioned arose as secondary 

 vesicles. It remains continuous with the lateral ventrit'les by the two inler-vcntricular foramina, 

 known also as ihe foramina of monro, one into ea(!h c(;n!l)ral hemisphere. The fourth ventricle 

 of the adult represents tlie cavity of the i)osterior primary vesicle and comes to lie between 

 the cerebellum and medulla ol)iongata, since the cerebellum likewise extends posteriorly from 

 its region of origin, "^riie cavity of the middle i)rimary vesicle becomes the cerebral aqueduct, 

 or aqu('(hu't of Sylvius, passing under the corpora (juadrigemina antl connecting the fourth or 

 posterior ventricle witli the third. 



Development of the nerve fibres. — All axones begin as outgrowths or processes of the cyto- 

 plasm of neuroblasts. Most of such i)rocesscs are sent out at a very early stage in the develop- 

 ment of the nervous system and extend to the tissues they are to innervate when these tissues 

 are as yet quite near the neural tube. Then, as the structures of the body elaborate and assume 



