THE MENINGES. 3 n 



strable interlacing fibres, with fusiform enlargements and elongated nuclei, 

 none can be identified as nerves, while of the numerous cells which the lobe 

 contains, only a few of large size and pigmented cytoplasm uncertainly 



Wall of diencephalon 



Posterior (cerebral) lobe 

 Anterior (oral) lobe 



Wall of oral cavity 



FIG. 358. Portion of sagittal section of rabbit embryo, showing developing pituitary body ; alveoli are 

 sprouting from wall of oral diverticulum. X 50. 



resemble nervous elements. With the exception of neurogliar cells, the 

 existence of definite nervous tissue within the cerebral lobe of the mature 

 human hypophysis is very doubtful. 



THE MENINGES. 



The entire cerebro-spinal axis is surrounded by three membranes, or 

 meninges. These are: (i) an external dense fibrous membrane, the dura 

 mater, which is closely attached directly to the inner surface of the skull and 

 in the vertebral canal forms an independent loose sheath for the spinal cord; 

 (2) an internal connective tissue tunic, the pia mater, which contains the 

 blood-vessels supplying the nervous tissue and, therefore, is adherent to 

 every part of the free surface of the brain and spinal cord; and (3) an inter- 

 mediate delicate nonvascular membrane, the arachnoid, which usually lies 

 close to the dura and varies in its relation to the pia mater. 



Between the dura and the arachnoid lies a narrow, for the most part 

 capillary cleft, the subdural space, which contains a small quantity of clear 

 straw-colored fluid, of the nature of lymph. The arachnoid and the pia 

 mater are separated by a much larger cavity, the subarachnoid space, which 

 in certain places, especially on the basal surface of the brain, reaches exten- 

 sive dimensions. It contains the cerebrospinal fluid, that is usually limpid 

 or slightly yellowish, and may show a very few lymphocytes (estimated 

 normally as 5 cells per cubic millimeter of fluid). The cerebrospinal 

 fluid is produced within the brain-ventricles, from the tufts of blood-vessels 

 of the several choroid plexuses, and, after filling the ventricular spaces and 

 the central canal of the cord, escapes through the thin roof of the fourth 

 ventricle into the subarachnoid spaces. 



