CIO 



BRAIN. 



BRAIN. 



650 



office ; it supplies the place of the periosteum to the inner surface of 

 the bones of the cranium, sustaining their nutrient vessels ; and it 

 serves as a defence to the brain, and a support to the different masses 

 into which it is divided. 



The dura mater gives off several elongations or productions called 

 processes, which descend between certain portions of the brain ; 

 the most remarkable of which is termed the superior longitudinal 

 process, which extends from the fore to the back part of the skull 

 between the lateral halves of the cerebrum. Narrow in front, it 

 becomes gradually broader as it passes backwards, bearing, as has 

 been conceived, some resemblance in shape to a sickle or scythe, 

 whence the common name of it, falx cerebri. 



Where the falx cerebri terminates behind, there proceeds a large 

 lateral expansion of the same membrane, extending across the back 

 part of the skull beneath the posterior parts of the cerebrum, and 

 forming a complete floor or vault over the cerebellum. This mem- 

 branous expansion is called tentorium, the obvious use of which is to 

 prevent the cerebrum from pressing upon the cerebellum ; while from 

 the middle of the tentorium proceeds another membranous expansion, 

 which descends between the lobes of the cerebellum, and terminates 

 insensibly at the edge of the foramen magnum, performing for the 

 cerebellum the same office as the falx performs for the cerebrum : 

 hence it is called falx cerebelli. 



Moreover the component fibres of the dura mater in certain parts 

 of ita course separate into layers, which are so disposed as to leave 

 spaces between them, for the most part of a triangular form. These 

 triangular spaces, which are commonly termed sinuses, are lined by a 

 smooth membrane perfectly analogous to that which lines the veins 

 in the other parts of the body, and these sinuses perform the office 

 of veins, returning the blood from all the parts of the brain to the 

 neck. Nothing analogous to this structure occurs in any other part 

 of the venous system. In almost every other part of the body the 

 pressure of surrounding parts is a most important aid to these 

 vessels in enabling them to carry on the circulation of the blood ; 

 but in the brain the venous tubes are guarded from pressure, the 

 dense dura mater being for this purpose stretched so tensely over 

 them that the weight of the surrounding parts is completely taken 

 off them. 



The smooth surface of the brain which is exposed on the reflection 

 of the dura mater, is formed by its second investing membrane which 

 is named the Tunica Arachnoidea, from the extreme tenderness and 

 delicacy of its tissue, which gives it a resemblance to a spider's web. 

 This thin colourless and transparent membrane is spread uniformly 

 over the surface of the brain, covering all the eminences termed 

 convolutions (fig. i. 2, 2), but not insinuating itself between any of 

 the depressions between the convolutions. (Fig. iv. 7.) On account of 

 its extreme tenuity and ita close adhesion to the membrane beneath 

 it, it cannot be easily separated from the latter ; but there are 

 situations at the basis where the arachnoid membrane, as it passes 

 between opposite parts of the brain, can be seen distinct from the 

 subjacent tunic. 



The third investing membrane, the Pia Mater, derives its name 

 like the former from the tenderness and delicacy of its tissue ; but 

 unlike the tunica arachnoidea, in which not a single blood-vessel 

 has hitherto been described, the pia mater is exceedingly vascular. 

 The blood-vessels with which every part of this delicate membrane 

 is covered are the nutrient arteries of the brain ; before they pene- 

 trate the brain these vessels divide, subdivide, and ramify to an 

 extreme degree of minuteness upon the external surface of this 

 membrane, so that the blood does not enter the tender cerebral 

 substance with too great force. When a portion of the pia mater is 

 gently raised from the brain, these blood-vessels appear as exceedingly 

 fine delicate threads, which on account of the elasticity with which 

 they are endowed are capable of elongation as they are drawn out of 

 the cerebral substance. As the pia mater contains and supports the 

 nutrient vessels of the brain, this membrane is not only spread as a 

 general envelope over ita entire surface, but it penetrates between all 

 its convolutions, and lines every cavity which is formed in it. 



It has been stated that the large portion of the cerebral mass, 

 termed the cerebrum, occupies the whole of the upper part of the 

 cavity of the cranium. The cerebrum is divided into two equal 

 lateral halves termed hemispheres (fig. i. 2, 2), which have an ovoid 

 figure somewhat resembling an egg cut longitudinally into two equal 

 parts. The hemispheres are separated from each other by the 

 membrane already described, the falx cerebri (fig. I 3) ; and their 

 inner sides in apposition with the falx are flattened, while their upper 

 and outer surfaces are convex, being accurately adapted to the 

 concavity formed by the inner surface of the bones of the cranium. 



Each hemisphere is subdivided into an anterior, a middle, and a 

 posterior lobe, but it is only on the under surface of the brain that 

 these lobes are accurately denned. (Fig. ii. 1, 2, 3.) The anterior and 

 middle lobes are separated from each other by a deep fissure named 

 the fisaura sylvia (fig. ii. 4), which extends obliquely backwards from 

 the basis to a considerable depth between the convolutions ; but the 

 ini'lille is distinguished from the posterior lobe, not by a fissure but 

 by a superficial excavation on the under surface of the posterior lobe. 

 (Fig. ii. 5.) The anterior lobes rest upon the orbital plates of the 

 frontal bone; the middle lobes are lodged in the temporal fossa; 



formed by the sphenoid and temporal bones, while the posterior lobes 

 are supported upon the tentorium. 



The whole of the external convex surface of the hemispheres is 

 divided into numerous eminences termed convolutions, which run in 



Fig. I. Upper Surface of the Brain. 



1, Cut edge of the bones of the cranium ; 2, superior convex surface of the 

 two hemispheres of the cerebrum, with their convolutions ; 3, separation between 

 the two hemispheres of the cerebrum, occupied by the falx cerebri. 



-cl 



i 



Fig. II. Base of the Uniiu. 



1 , Anterior lobes of the cerebrum ; 2, middle lobes of the cerebrum ; 3, pos- 

 terior lobes of the cerebrum ; 4, fissure separating the anterior from the middle 

 lobes, named the fissura Sylvia ; 5, situation of the superficial excavation 

 forming the boundary between the middle and the posterior lobes ; G, the two 

 hemispheres of the cerebellum, composed of flattened laminrc or layers; 7, tho 

 medulla oblongata, which, in this position of the brain, rests upon and covers 

 the vermiform process ; 8, corpora pyramidalia ; 9, corpora olivaria ; 10, tuber 

 annulare, or pons varolii j 11, dccussation of the corpora pyramidalia; 

 a t ft, r, it, cerebral nerves. 



different directions, and are of different sizes and lengths in different 

 parts of the hemisphere. (Fig. i. 2, 2.) The depressions or fissures 

 between the convolutions, termed clefts, or sulci, generally penetrate^ 

 the consistence of the brain to the depth of about an 'inch or an 

 inch and a half. (Fiy. iv. 7.) The greater number of these pursue a 



