22 APPLIED ANATOMY. 



tissue pass to the pia mater ; the space between the fibrous layer of the arachnoid above 

 and the pia mater and convolutions of the brain below is called the subarachnoid 

 space. This is a lymph space and contains the cerebrospinal fluid! This fluid is 

 normally about 60 c.c. (2 ounces) in quantity, but in injuries to the brain in which 

 the subarachnoid space is opened, the fluid is secreted and discharged very rapidly. 



As has already been mentioned, the arachnoid sends a prolongation into the 

 internal auditory meatus, hence a fracture through it would open the subarachnoid 

 space. This space communicates with the ventricles of the brain through three 

 openings in the pia mater at the lower back portion of the roof of the fourth ventricle ; 

 these are called the foramina of Magendie, Key, and Retzius. The cerebrospinal 

 fluid extends down the spinal canal and can be removed by tapping with a trochar, as 

 is practised in the lumbar region. 



The Pia Mater. The net-work of vessels, with their supporting membrane, 

 which covers the convolutions of the brain, forms the pia mater. The fibrils of con- 

 nective tissue supporting the vessels are attached to the fibrous layer of the arachnoid 

 above, so that the pia and arachnoid are in reality continuous structures. The spaces 

 between these fibrils are often quite large and communicate with each other, forming 

 the subarachnoid space. The lower portions of these fibrils are united and form a 

 basement membrane which lies directly on the convolutions of the brain and dips into 

 the sulci. The blood-vessels are intimately connected with this lower pial membrane 

 and not with the arachnoid above. These vessels penetrate into the substance of the 

 brain, carrying with them a covering or sheath of pia mater. This is called the peri- 

 vascular lymph sheath and, of course, communicates with the subarachnoid space 

 above. These vessels nourish the brain. The perivascular lymph sheaths are also 

 said to form capsules around the great pyramidal and large glial cells of the cortex. 



AFFECTIONS OF THE MEMBRANES OF THE BRAIN. 



Both the dura mater and the pia mater are subject to inflammation and hemor- 

 rhages. The arachnoid being practically a part of the pia mater is involved in its 

 diseases, so that no mention is made of it as being separately affected. 



Inflammation of the Dura Mater; Pachymeningitis. The outer surface 

 or inner surface of the dura may be involved, constituting pachymeningitis externa 

 or interna. 



Pachymeningitis Externa. The external surface is most often affected by 

 injuries from without, or by extension of diseases from the adjoining bone. In cases 

 of fracture the inflammation which accompanies healing frequently causes the dura 

 to become densely adherent to the overlying skull. This is noticed particularly when 

 trephining operations are performed for the relief of focal or Jacksonian epilepsy. 

 Should the fracture be compound or open, the occurrence of sepsis will tend to 

 involve the adjacent dura mater. The same occurs in cases of necrosis. Syphilitic 

 disease of the bones is most apt to affect the vault of the skull, while the dura 

 towards the sides and base is most often involved by suppurative ear disease. The 

 dura also becomes involved in tumors and gummata. 



Inflammation of the dura is not apt to be a marked disease. It is a very dense 

 membrane with few blood-vessels, therefore it is quite resistant to inflammatory proc- 

 esses. It acts as a barrier to the farther extension of an inflammation rather than 

 as a carrier. Therefore we see epidural collections of pus existing for a considerable 

 time without brain symptoms supervening. 



The dura mater contains the large cerebral venous sinuses, and when the inflam- 

 matory process occurs in those regions, the sinuses become inflamed and thrombosis or 

 clotting occurs. The clot becoming infected breaks down, the pus and debris pour into 

 the general circulation, and general septicaemia, and even death, is caused. This is most 

 liable to occur in the region of the ear, where the infection is apt to reach and involve 

 the lateral (transverse) sinus. Infection of the longitudinal sinus is much more rare. 



Pachymeningitis interna is an inflammation of the inner surface of the dura. It 

 occurs, to a certain extent, in cases of gumma or other new growths involving the inner 

 surface of the dura or extending from the pia mater below. The name pachymeningitis 

 interna, also called haemorrhagica, is usually restricted to a chronic inflammation of 



