414 Anatomy Applied to Medicine and Surgery. 



sphenoidal lobes, is another distinct interval or space, and 

 these two intervals or spaces are connected with one an- 

 other around the sides of the medulla oblongata. 



This subarachnoidean space contains the cerebro-spinal 

 fluid, and the larger divisions of the space, above men- 

 tioned, at the base, allow the brain to rest, as it were, on a 

 sort of water-bed, thus minimizing the effects of jars, 

 blows, falls, etc.. The cisterna magna or posterior space, 

 i.e., the portion of the subarachnoid space behind the me- 

 dulla, communicates, by means of three small openings 

 (the chief one of which is the foramen of Majendie) in 

 the pia mater covering the fourth ventricle. By means 

 of these foramina, the ventricles of the interior of the 

 brain, that are distended in such diseases as hydrocephal- 

 us, etc., may unload themselves into the subarachnoid 

 space, and the increased quantity of fluid, passing down 

 into the subarachnoidean space of the coverings of the 

 cord may be better accommodated in the vertebral canal 

 than in the cranium, since the soft tissues joining the verte- 

 brae together permit more or less distention. It is pos- 

 sible that the Pacchionian bodies may act as an outlet for 

 increased pressure in the cerebral sinuses, since they con- 

 sist of a central core of subarachnoidean tissue and are 

 covered with arachnoid membrane as well as with a layer 

 of dura mater, and, inasmuch as they project into the 

 longitudinal sinus, and, since injections thrown into the 

 subarachnoidean space find their way into the sinus, it is 

 possible that, conversely, they may offer an outlet from 

 the congested venous circulation into the subarachnoid 

 space. 



The pia mater is a delicate vascular membrane and 

 serves the purpose of conveying to the interior of the brain 

 the finer bloodvessels of the larger trunks which have 

 divided freely in the subarachnoid space. 



