210 ANATOMY AND PHYSIOLOGY FOR NURSES 



web. It is a very delicate, serous membrane, and, like 

 all other serous membranes, is a closed sac one part, form- 

 ing the parietal layer, lines the under surface ; the other, or 

 visceral layer, is reflected over the brain. 



This membrane is perfectly smooth, and is sufficiently 

 lubricated to prevent friction. It is in close connection 

 with the pia mater. At the base of the brain it forms 

 spaces, known as subarachnoidean spaces, which contain 

 the cerebrospinal fluid; this fluid serves to protect the brain 

 and nerve-centers from sudden shocks and concussions, 

 such as blows, as well as loud noises. The base of the 

 brain is supported by a bed of water. In fractures at the 

 base of the skull or in the temporal region water will be 

 seen to ooze through the ear and along the line of the 

 fracture; this symptom is generally considered as proof 

 of the presence of a fracture. 



The Pia Mater. This is the third membrane that 

 invests the entire surface of the brain, and forms the 

 velum interpositum and choroid plexuses of the third and 

 fourth ventricles. It is extremely vascular, its blood- 

 supply being derived from the internal carotid and ver- 

 tebral arteries, and forms a bed in which the blood-vessels 

 divide and subdivide before they enter the brain. It 

 dips in between the convolutions, as well as into the lateral 

 ventricles, to supply blood to the interior of the brain 

 substance. 



The great mass of nervous matter termed the brain 

 is, as we have said, divided into three great parts the 

 cerebrum, the cerebellum, and the medulla oblongata. 



The cerebrum is of oval form : it is divided in the middle 

 line, by the longitudinal fissure, into two parts, termed the 

 right and the left hemisphere. The surface of the hemi- 

 spheres is of tortuous character, and divided by furrows, 

 or sulci, into convolutions. The furrows serve to accom- 

 modate the vessels in their course to the sinuses ; they also 



