IIOO 



HUMAN ANATOMY. 



recess one of the pair of diverticula that overlie the inferior cerebellar peduncles 

 and add materially to the transverse dimension of the ventricle. After enclosing the 

 lateral recess the taenia leads to the stalk of the flocculus and the inferior velum. 



Within the triangular field of the teia chorioidea, the pia mater takes advantage 

 of the attenuation of the ventricular wall to effect imaginations by which its blood- 

 vessels apparently gain entrance into the ventricle. Such invaginations, known as 

 the choroid plexus of the fourth ventricle, occur in the ventricular roof 

 on each side and in the immediate vicinity of the mid-line, where they appear as 

 parallel villous or fringe-like stripes, the median plexus, which extends upward 

 from near the obex to the inferior medullary velum. Opposite the nodulus they 



FIG. 951. 



Nucleus gl 



Fourth ventricle 



Lateral recess of ventricle"*^ ^QSt 



Inferior olivary nucleus 



Choroid plexus 

 Posterior longitudinal fasciculus 



Pyramidal tracti 



Section across lower third of fourth ventricle, showing internal cerebellar nuclei, choroid plexus, lateral recesses and 

 medulla ; new-born child. X 3/i- Preparation by Professor Spilfer. 



diverge and, as the lateral plexuses, invaginate the wall of the lateral recesses. 

 The vascular complex lies within the fold of pial tissue, the space between the pial 

 layers being occupied by prolongations of the arachnoid. 



Notwithstanding its conspicuous thinness during the first half of fcetal life, the tela 

 chorioidea suffices to completely close the ventricle. From about the fifth month, 

 however, the delicate membrane is perforated by an aperture that remains throughout 

 life. This opening, the foramen of Magendie (apertura mcdialis vcntriculi quart!) 

 lies immediately above the obex and between the strands of the choroid plexus. 

 Two additional clefts, the foramina of Luschka (aperturac laterales), usually exist, 

 one on each side, in the wall of the lateral recesses in the neighborhood of the vago- 

 glosso-pharyngeal nerves. By means of these three openings, and probably by these 

 alone, the system of ventricular cavities and the central canal of the spinal cord are 

 brought into communication with the subarachnoid lymph-space. A path is thus 

 provide. 1 by u hich the cerebro-spinal fluid, secreted within the lateral, third and fourth 

 ventricles by tin- various rhoroid plexuses, constantly escapes and thereby prevents 

 undue accumulation and distension within the cavities of the brain and spinal cord. 



THE DEVELOPMENT OF THE HIND-BRAIN DERIVATIVES. 



In the general sketch of the development of the brain previously given (page 1061), it was 

 pointed ,,ut that the hind-brain, or rhombcnrcphalon, includes two subdivisions, the myclfHceph- 

 alon and the wff,-m;-f>hii/on, the extreme upper part of the latter being designated the isthmus. 

 It has be<-,i further noticed that tV junction of the cord and brain-segments of the neural tube 

 corresponds with the conspicuous cervical flexure, whose early appearance is followed by an 



