NERVOUS SYSTEM. (NERVOUS CENTRES. ABNORMAL ANATOMY.) 



717 



The existence of serous fluid in the arach- 

 noid cavity is of very rare occurrence. In some 

 instances old adhesions of the two layers of 

 arachnoid to each other circumscribe a space in 

 which fluid accumulates. 



Blood is sometimes effused into the subarach- 

 roid cavity. This is frequently the case in 

 injuries of the head, the blood escaping from 

 broken vessels of the pia mater. Sometimes 

 the blood effused into either lateral ventricle 

 will escape into the subarachnoid cavity, break- 

 ing down the membrane of the ventricle. If an 

 apoplexy occur near the surface of the brain, 

 the laceration of the cerebral substance may 

 extend quite to the surface, and the blood may 

 pass through the pia mater into the subarach- 

 noid space. 



In some instances we find blood in the 

 cavity of the arachnoid (the arachnoid sac). 

 The blood is either loose in the sac, or it is 

 more or less closely connected with the inner 

 surface of the membrane lining the dura mater. 



In a recent communication from Mr. Pres- 

 cott Hewitt, published in the last volume of 

 the Medico-Chirurgical Transactions, the prin- 

 cipal facts relating to this subject have been 

 collected and arranged in an interesting form. 

 Mr. Hewitt describes these effusions of blood 

 as existing in four forms : 



" 1. The extravasated blood may be either 

 liquid or coagulated ; if in the latter state, it 

 may be in clots, or spread out in the shape of a 

 thin membranous layer, covering a greater or 

 less extent of the surface of the brain. 



" 2. Sometimes the extravasation presents 

 itself under the shape of a false membrane, 

 possessing more or less of the original colour of 

 the blood. 



" 3. The blood may be fixed to the free 

 surface of the arachnoid and there maintained 

 by a membrane, which to the naked eye pre- 

 sents all the characters of the serous membrane 

 itself. 



" 4. The blood is frequently found enclosed 

 in a complete cyst of various degrees of thick- 

 ness, which may be removed unbroken from 

 the cavity of the serous membrane. 



" The lour divisions above referred to," adds 

 Mr. Hewitt, " may be and often are combined 

 with each other, but in whatever state the extra- 

 vasated blood has been found, it has, in the 

 majority of cases, corresponded to the upper 

 surface of the brain, and has been rarely met 

 with in the cerebellar fossae." 



It is impossible that these effusions of blood 

 can have any other source but the minute 

 bloodvessels of the pia mater or the dura mater, 

 which becoming ruptured allow the blood to 

 burst through the serous membrane by whicli 

 they are covered. They occur mostly in per- 

 sons of a scorbutic or hemorrhagic habit, or 

 in whom the arteries have become brittle from 

 abnormal deposits in them; and it is not im- 

 probable that whilst the imperfect nutrition of 

 the arteries is going on, the serous membrane 

 itself suffers, becomes wasted, and therefore 

 easily yields to the force of the blood as it 

 escapes from the bloodvessels. 



Pus is found in the subarachnoid cavitv 



where there has been inflammation of the pia 

 mater and arachnoid, and more rarely in the 

 arachnoid sac. 



Of the pia mater. This membrane being 

 the vascular membrane of the brain, and con- 

 taining the nutrient vessels as well of the sur- 

 face of the brain as of the visceral layer of the 

 arachnoid, is the seat of all those changes in 

 the condition of the bloodvessels or of their 

 contents, which give rise to, or are caused by, 

 morbid states either of the nervous matter or 

 of the serous membrane. 



All those changes which indicate hyperaemia 

 or anaemia of the convolutions of the brain 

 occur in the pia mater ; and the colour of this 

 membrane will vary according to the quantity 

 of blood contained in its bloodvessels. 



There are no definite signs which enable the 

 anatomist to pronounce whether an hyperaemia 

 be of the active and inflammatory kind, or 

 passive, and dependent on some cause remote 

 from the brain itself, or even upon a post- 

 mortem cause, unless it be accompanied with 

 those undoubted products of the inflammatory 

 process, pus or lymph. 



A highly injected state of the vessels of the 

 pia mater will frequently be caused by the 

 manner of the patient's death. Where the 

 respiratory actions have been laboured and dif- 

 ficult prior to death, this is sure to occur: we 

 find it also when death has been caused by 

 asphyxia, however produced. 



In convulsive diseases the pia mater and the 

 whole brain become highly injected more as a 

 consequence of the impeded circulation caused 

 by the struggles of the patient interfering with 

 the due exercise of the respiratory movements, 

 than as the cause of the convulsions. Indeed, 

 there seem good grounds for believing that con- 

 vulsions are more frequently caused by a de- 

 ficient supply of blood to the brain than by a 

 superabundant flow of it to that organ. 



The pia mater is the seat of the principal 

 morbid deposits which affect the brain. Of 

 these tubercle is among the most common ; 

 it most frequently occurs on the surface of the 

 convolutions ; but it may be found wherever 

 the pia mater exists, either in the interior or on 

 the outside of the brain. It occurs less fre- 

 quently on the pia mater of the cerebellum 

 than in any other situation. 



The tubercular deposit in the pia mater com- 

 mences by the developement of minute granu- 

 lations of a grey, clear, or semi-transparent 

 material. These are deposited close to each 

 other over a greater or less surface, forming a 

 group, and several such groups may be formed 

 near each other. After a time this grey ma- 

 terial is changed into a yellow granular matter, 

 which is sometimes enclosed in a cyst. 



Tubercular matter originally deposited on 

 the surface of the pia mater in the sulcus of a 

 convolution may have the appearance as if it 

 had been formed in the substance of the brain. 

 The sulcus is obliterated, and the tubercle, 

 enlarging towards the brain, becomes, in a 

 short time, surrounded by cerebral matter. 



Sometimes tubercle deposited in some part 

 of the pia mater excites inflammation in the pia 



