GENERAL DEVELOPMENT OF THE BRAIN. 



1063 



corpus callosum, the fundamental relationships indicated by embryology are of such 



value that, e\'en in the description of the adult organ, grouping of the various parts 



of the brain upon a develop- 



FiG. 914. 



Epithalamus 



Thalamus 



Pallium 



Corpus 

 striatum 



mental basis is found advan 

 tageous. Although strict 

 adherence to such a plan 

 would be at times inconven- 

 ient, and, therefore, will not 

 be followed, constant refer- 

 ence to primary relations is 

 imperative. It will be con- 

 venient, therefore, at this 

 place, to call attention to 

 the accompanying outline 

 diagrams which illustrate 

 the principles established by 

 His in his epoch-making 

 studies of the human brain. 

 In addition to showing the 

 five cerebral vesicles, F'ig. 

 913 indicates the relative 

 position and extent of the 



two fundamental subdivisions of the lateral walls of the neural tube, the dorsal 

 or alar and the ventral or basal laminae, which play such important roles in the 

 differentiation of the various parts of the brain-stem. Fig. 914 shows a later 

 stage, in which the genetic relations of all the more important parts of the brain may 

 be recognized. The greatest complexity is presented in the development of the 

 derivations of the fore-brain, particularly of those which are differentiated from the 

 diencephalon and later are found connected with the third ventricle. In order to 

 set forth the developmental relations of the fore-brain, the following table from His. 

 slightly modified, will be of service : 



(Pallium 



( HemisphseriuAi :'i. '. Corpus striatum 



\ ■'".'' ( Rhinencephalon 



Rhinencephalon 



Pars optica 

 hypothalami 



Metathalamus 

 Pars mammillaris hypothalami 

 Mesencephalon 



Pedunculi cerebri 

 Isthmus 

 Cerebellum 

 Pons 



Medulla 



Dorsal zone 

 Ventral zone 



Diagram showing chief derivatives from cerebral visicles ; based on 

 brain of embryo of third month. (His.) 



Fore-Brain 



or 



Prosencephalon 



r Telencephalon -i 



I (^ Pars optica hypothalami 



Id. 



Pars mammillaris hypothalami 



\ Thalamus 



ENCEPHALON 



.Thalamencephalon , 



Epithalamus 

 Habenula 

 ■! Corpus pineale 



Commissura post. 

 Metathalamus 



Corpora geniculata 



Parts of the Brain derived from the Rhombencephalon. 

 THE MEDULLA OBLONGATA. 



The medulla oblongata, sometimes called the didd and usually designated by the 

 convenient but indefinite name ' ' medulla, ' ' is the direct upward prolongation of the 

 spinal cord. It begins at the decussation of the pyramids below, about on a level 

 with the lower border of the foramen magnum, and ends at the lower margin of the 

 pons above and is approximately 2.5 cm. (i in) in. length. Its general form is 

 tapering, increasing in breadth from the transverse diameter of the cord (10 mm.) 

 below, to almost twice as much (18 mm.) above, and in the antero-posterior dimen- 

 sion from 8-15 mm. Its long axis corresponds very closely with that of the cord and 

 is, therefore, approximately vertical. The medulla, surrounded by the pia and arach- 

 noid, lies behind the concave surface of the basilar portion of the occipital bone, with 

 its dorsal surface within the vallecula between the hemispheres of the cerebellum. 



Superficially, in many respects the medulla appears to be the direct continuation 

 of the spinal cord. Thus, it is divided into lateral halves by the prolongation of the 

 anterior and posterior median fissures ; each half is subdivided by a ventro-lateral 

 and a dorso-lateral line of nerve-roots into tracts that seemingly are continuations of 



