CHAP. X.] THE CEEEBRO-SPINAL FLUID. 253 



of fibrous tissue, which interlace slightly, and are most abundant 

 in the cervical region. Along the posterior surface of the spinal 

 cord, in the middle line, the sub-arachnoid space is divided by means 

 of a septum, which is probably only a modified portion of the tissue 

 of the pia mater. This septum is most perfect in the dorsal region, 

 but in the lumbar and cervical regions it is cribriform, and in some 

 parts is very difficult of demonstration. Dr. Sharpey regards it as 

 the reflection along the median line of a serous membrane, which 

 he supposes to line the sub-arachnoid cavity. Did such a mem- 

 brane exist, we should find an epithelium, which, however, we have 

 sought for in vain. 



The connexion of the arachnoid to the subjacent pia mater is 

 not so loose in the head as in the spine. On the superior and 

 lateral surfaces of the brain, where the convolutions are most pro- 

 minent, the adhesion is very close, but opposite the sulci between 

 the convolutions the pia mater recedes from the arachnoid and sinks 

 to the bottom of each fissure, leaving large areolse in which fluid 

 may accumulate. Along the fissure of Sylvius, at the base of the 

 brain, between the cerebellum and the posterior surface of the me- 

 dulla oblongata, and between the posterior edge of the corpus 

 callosum and the superior surface of the cerebellum, the arachnoid 

 and pia mater are very loosely connected, so that at these situa- 

 tions spaces are found which are favourable for the accumulation 

 of fluid. 



The Cerebro-spinal Fluid. This fluid, which fills the sub-arach- 

 noid space during life, keeps the opposed surfaces of the arachnoid 

 membrane in intimate contact. Its quantity, which varies between 

 two and ten ounces, is in the inverse ratio of the bulk of the 

 brain and spinal cord. Thus it is most abundant in old persons in 

 whom these organs have shrunk, and it accumulates in cases of de- 

 ficiency of any portion of them from malformation or disease. Its 

 presence seems necessary to the healthy action of the nervous cen- 

 tres, for the removal of it in dogs by Majendie caused considerable 

 disturbance of their functions, probably by favouring distension of 

 the blood-vessels. It is, however, capable of being regenerated as 

 quickly as the aqueous humour of the eye, and its reproduction re- 

 stores the nervous centres to their natural state. When removed 

 from the body a few moments after death, this fluid is, according to 

 Majendie, remarkably limpid ; it has a sickly odour and a saltish 

 taste, and is alkaline, restoring the colour of reddened litmus. 



The cerebro-spinal fluid is most probably secreted by the pia 

 mater, since it is found wherever that membrane and sufficient 



