BRA IX. 



BRAIN. 



_ course, but com* run longitudinally, others obliquely ; some 

 communicate with each other, while other* terminate separately in 

 the MibsUDOO of the brain. (Fiy. iv. 7.) 



The nervous nutter constituting the cerebrum in compoied of two 

 distinct subctances, which differ from each other materially both in 

 their colour and consistence. (Fig. iv. 7.) Tho outer substance is 

 sometimes termed nutritious, from iU boing of a grayish-brown 

 colour ; at other time* cortical, from iU nut-rounding the inner part 

 of the brain, u the bark the inner parts of the tree ; by some it 

 u ah called glandular, and by others secretory, from the supposition 

 that its nature in that of a gland, and that it secretes a peculiar 

 fluid. It u of a softer consistence than the inner part, and leaves by 

 denotation a smaller quantity of solid residuum. It U composed 

 almost entirely of blood-Teasels connected and sustained by exceed- 

 ingly fine cellular membrane. Its structure is uniform throughout, 

 presenting no appearance whatever of a fibrous texture. It gives to 

 the entire surface of the cerebrum an external covering, generally 

 about the tenth of an inch in thickness. (Fig. iv. 7.) 



The inner substance, termed white or medullary (Jig. iv. 7), is firmer 

 in consistence and larger in quantity than the gray matter ; anil when 

 an incision is made into it, its surface is spotted with red point*, the 

 cut orifices of its vessels, which vary in number and size according ai 

 they may be more or leas distended with blood. It is now universally 

 agreed that this part of the brain U composed of fibre*. When 

 examined in its recent and most perfect state, especially after it has 

 been artificially hardened and condensed by the action of heat or 

 certain chemical substances, if it be carefully scraped with a blunt 

 instrument these fibres become perfectly distinct and are of consider- 

 able magnitude, with furrows between them, which for the most part 

 are placed in such a direction at* to converge towards the base of the 

 brain. (Pig. iv. 6, 5, 4.) The fibres do not merely unite, forming what 

 are called commissures, but they actually cross each other and p;uw 

 into the opposite (idea of the body. This decussation of the medullary 

 fibres has been demonstrated in the most satisfactory manner by 

 l>r-. Oall and Spurzheim. 



The cerebellum in situated at the basis of the cerebrum towards 

 it* posterior part. (Fig. ii 6, 6.) IU form is elliptical, its largest 

 diameter extending transversely from one side to the other. (Pig. ii. 6.) 

 Like the cerebrum it is divided into two lateral halves or hemispheres 

 (Jig. ii. 6), which are separated by the falx cerebellL In the centiv of 

 its upper surface there is a distinct prominence, termed the vermiform 

 process (Jig. ii. 7), which may be considered as the fundamental part 

 of the organ, becaus.-, in the lower animals, whatever other parts of 

 the cerebellum are absent, this ia invariably present, affording thus 

 the nucleus or rudiment of the organ, from which, by the addition of 

 other parts, as the hemispheres or lateral lobes, &c., the mure perfect 

 organ of the higher animal is built up. 



The external surface of the cerebellum is divided into flattened 

 strata or layers (fig. ii. 6), separated by fissures which correspond to 

 the clefts or sulci between the convolutions. The pia mater, bearing 

 the nutrient arteries of the cerebellum, passes between every one of 

 these fissures ; while the arachnoid membrane is simply extended 

 over them. If n vertical section be made through either heniispli. n 

 of the cerebellum, a thick maw of white substance U seen in the 

 centre, which, as it divides into the several strata, presents an arbor- 



f 



Kilt. HI. Vertical Section of the Brain. 



1 , Bundles of medullary fibre* In the central part of the nrrvoui apparatus ; 

 9, white matter taming the centra of the fundamental part of the cerebellum ; 

 3, Teitical wctlon of the cerebellum, ihowinit Hie arboretccnt arrangement of 

 lu component lamina-, and forming the appearance called arbor vitro ; 4, ultua- 

 tion of the third ventricle; A, fibre* of white matter, forming the upturn 

 laeidam, the medullary layer which ntparatc* the two lateral ventricle* from 

 each other ; 6, nbre* of white matter, forming tlie corpui callosum, Immedi- 

 ately beneath which are situated the lateral ventricle* ; 7, convolution* of the 

 cerebrum. 



csoent appearance commonly denominated the arbor vita;. (Fiy. iii. 8.) 

 These strata diverge towards the circumference of the cerebellum, 

 and are covered externally by gray substance. (Fly. iii. 3.) 



In front of the cerebellum is placed a large mas* of nervous matter, 

 forming a very considerable eminence, commonly termed the tuber 



annulare, or the pons varolii. ( l'< : t ii. 1 1 1. ) The external surface of this 

 body U convex, and it is ilivi.l. .1 int.. two lateral halves by a mi'i.lle 

 groove. ( Fig. ii. 10.) It in joined to the cerebrum by two thick white 

 cords named the crura cerebri, and to the cerebellum by two similar 

 cords named the crura cerebelli. The crura cerebri are continur<l 

 (from the tuber) outwards and forwards to tin- under :m<l middle 

 part of each hemisphere of the cerebrum, in which they are lost. In 

 like manner the crura cerebelli are COM inn. d < >utwards and backwards 

 into the hemispheres of the onrebcllnm. in wlii.-h they terminate. 



The Medulla Oblongata is that portion ..I' the cerebral mas* which 

 intervenes between the tuber annulare and the foramen magnum 

 (Jiy. ii. 7) : beyond the foramen magnum it take* the name of spinal 

 cord. On the anterior surface of the medulla oblongata there are 

 four eminences contiguous to each other. (Fiy. ii. 7.) The two internal 

 are named corpora pyniiui.i.ili.i. or tin- pyramids (Jiy. ii. 8) ; and the 

 two external the corpora olivaria (Jiy. ii. Ii), or the olivary bodies. 



If the membranes which invest the medulla oblongata are carefully 

 removed, and its middle groove be gently drawn asunder, there will 

 be discovered four or five bands of white substance ascending obliquely 

 from one side of the medulla to the other. (Fig. ii. 11.) These bands 

 on each side decussate, some of them passing above and others below 

 those of the other side, so that they are interwoven like platted straw. 

 (Fig. ii. 11.) These bands are named the decussating bands of the 

 corpora pyramidalia, and their decussation is conceived to explain the 

 phenomenon familiar to the physician and surgeon, that when injury 

 is done to one side of the brain a consequent disturbance of function 

 ia manifested on the opposite side of the body. 



Taken as a whole, the nervous mass constituting the brain is strictly 

 symmetrical, that is, the different parts of which it is composed are 

 so arranged, that, if the organ be supposed to be divided into two 

 lateral halves by a plane passing perpendicularly through its centre, 

 the parts placed on each side of this plane have a perfect corresponil- 

 ence with each other, and form in fact reduplications of each 

 ( /'<;/. ii.) The principal parts of the cerebral mass are thus <1< 

 but they are all united on the median line with their fellows of the 

 opposite side. This union is effected by medullary bands of various 

 sizes and figures which pass from one to another, called commissures. 

 Thus the double parts of the cerebellum are united by means of the 

 large moss of cerebral matter already sjioken of under the name of 

 tuber annulare, or pons varolii. ( Fig. ii. 10.) The hemispheres of the 

 cerebrum are united chiefly by a broad expansion of medullary matter, 

 which extends transvcr.-ely across from the bottom of one hemi-; 

 to that of the opposite side, called the corpus callosum, or the great 

 commissure of the bruin. ( /'/. iii. C, 6.) There are other connecting 

 bands of smaller size by which minor portions of the cerebral mass 

 are placed in communication, into a description of which it is not 

 necessary to enter here. 



The cerebral parts arc separated from one another at certain places, 

 and the intervals form cavities which are termed ventricles. Of these 

 ventricles there are commonly enumerated four, all of which are in 

 communication with each other. By far the largest of these air 1 1..- 

 two great cavities called the lateral ventricles, which are situated in 

 the interior of the hemispheres of the cerebrum. ( 'oninn-ncing in the 

 fore part of the anterior lobes, these cavities proceed backwards in a 

 direction parallel to each other through the middle into the posterior 

 lobes. Their figure is winding and exceedingly irregular, and they 

 are separated from each other b\ a tender moss of medullary matter 

 termed the septum lucidum. (Fig. iii. 5.) They are lined throughout 

 by a fine transparent membrane, which secretes a fluid that keeps 

 them moist, gives them a bright polished appearance, and pi 

 them from uniting. Thin membrane is the pia mater, which 

 tinned from the exterior surface of the brain hit ! ior cavities; 



and some anatomists describe the are -hnoiil membrane .;.; ar< omp.iny- 

 iug the pia mater in all its course through the ventricles. 



The middle or third ventricle is a vertical fissure between the two 

 large convex eminences called the thalami optici (>>/. iii. 4), situated 

 in the middle and back port of the lateral ventricles. The fourth 

 ventricle, called also ventricle of the cerebellum, is a cavity of con- 

 siderable extent situated between the cerebrum, the tuber annul. nv, 

 and the medulla oblongata. 



For a detailed account of the course of the fibres the reader is 

 referred to the work of Dra. Oall and Spur/.heim, entitled ' Kecherches 

 sur le Systcme Nerveux en general, et sur celui du Cerveau en par- 

 ticulier,' in which the direction of the cerebral fibres is not only 

 minutely and exactly described, but illustrated by excellent draw inn* 

 as Urge as the objects. The course of the fibres that compose the 

 pyramids (Jiy. ii. 8 and .if,'/- iv. 1) U as follows: Immediately before 

 their entrance into the tuber annulare the pyramids arc a little con- 

 tracted. ( Fig. ii. 8.) As soon as they enter this moss the pyramids are 

 divided into innumerable bundles of fibres (Jig. iv. 2), which are covered 

 by a thick layer of transverse fibres (Jig. iv. 2) that come from tlm 

 cerebellum. ( Fly. iv. 8.) These fibres of the pyramids, thus increased in 

 number, ascend, and receive at every point of their course fresh acces- 

 sions, until at their exit (from the tuber) forward and outward ihcy 

 form at least two-thirds of the crura cerebri, as is seen at fig. iv. 8. 

 Followed in their course forwards from .If//. >v. 3, they are manifestly 

 increased at every point by the accession of infinite numbers of 

 fibres. ( Pig. iv. 4.) At the point (Jig. iv. 5) the fibres, now exceedingly 



