THE MAMMALIAN NERVOUS SYSTEM 59 



third ventricle along the tcenia ventriculi tertii, or tcenia thalami, 

 and note also that the plexus itself is continued through the 

 interventricular foramen into continuity with the lateral cho- 

 roid plexus within the ventricle of the cerebral hemisphere. In 

 a specimen from which the septum pellucidum has been torn 

 away this continuity can be readily observed ; but do not at 

 this time attempt further dissection of the lateral choroid 

 plexus (see Section 132). 



The choroid plexuses of the third and fourth ventricles are 

 commonly referred to, as in the preceding paragraphs, as form- 

 ing the true roof of the brain cavity in these regions. But 

 according to the more precise usage of the B N A each of these 

 structures is really composed of two layers: (1) the lamina 

 epithelialis, which is the true -brain wall derived from the 

 embryonic neural tube, which here retains its embryonic 

 character as a non-nervous epithelium; (2) the pia mater, which 

 is here highly vascular and convoluted. In the B N A the 

 term plexus chorioideus is applied to this modified pia mater 

 alone. 



3. General Directions for Microscopic Material 



61. The further study of the spinal cord and brain stem can 

 best be carried out with both gross and microscopic material, 

 though good results can be obtained with either class of mate- 

 rial alone. If microscopic preparations are not available, it will 

 be found practicable to select from th? following paragraphs 

 those which apply to gross material only. Gross sections 

 through the human brain at the levels indicated below will 

 show many of the details referred to. 



62. Drawings of cross-sections. Make an outline sketch, 

 ventral s'de down, of each of four microscopic transverse sec- 

 tions ( Weigert method preferred) through the spinal cord taken 

 respectively from the cervical, thoracic, lumbar, and sacral 

 regions; and also select 6 to 12 transverse sections from the 

 series through the brain stem and similarly draw each of these 

 in outline. Make these outlines as accurately as possible about 

 six times natural size, or those from the spinal cord and lower 

 part of the medulla oblongata may be magnified 8 diameters 

 and those from the upper levels 4 diameters. The sketches of 



