653 



BRAMA. 



651 



numerous, manifestly assume a diverging course, proceeding in every 

 direction forwards, upwards, laterally, and backwards. (Fig. iv. 5, 6, 7.) 

 At length the radiating fibres, crossing and interlacing each other in 

 all directions, form an expansion or tissue, and, being folded in 



Tig. IV. Course of the fibres of the Brain. 



1 , Entrance of the anterior pyramids into 2, the tuber annulare, or pons 

 varolii ; 3, fibres of the pyramids much increased as they issue from the tuber 

 annulare ; 4, 5, continued increase in the fibres of the pyramids as they advance 

 onwards towards the convolutions ; 6, divergence of the fibres of the pyramids; 

 7, convolutions of the cerebrum, showing their depth, their gray matter, and 

 the sulci between them ; 8, cerebellum. 



various ways and covered with gray matter, constitute the convolu- 

 tions. (Fig. iv. 5, 6, 7,1.) Thus the pyramids, progressively increased 

 and developed, form a large portion of the anterior and middle lobes 

 of the cerebrum. If the corpora olivaria (Jiy. ii. 9) were traced in like 

 manner, they would be found to form the posterior lobes of the cere- 

 brum ; and the origin and source of the fibres constituting the main 

 bulk of the cerebellum can be demonstrated with the same clearness 

 and exactness. 



From the preceding account of the structure of the brain, which 

 shows it to be an exceedingly complex organ, it might have been 

 inferred from analogy that it would receive a large supply of blood ; 

 but the quantity actually sent to it is far greater than any analogy 

 could have led us to suppose. Haller made a calculation, from which 

 he concluded that one-fifth of all the blood sent out of the left ven- 

 tricle of the heart is carried to the head, yet the weight of the brain 

 in the human subject is not more than one-fortieth of that of the 

 whole body. Even if this estimate, which is generally thought too 

 large, be reduced to one-tenth, according to the idea of Monro, it will 

 still leave a very great over-proportion. There is no part of the 

 structure of the brain more curious than the various contrivances 

 connected with the circulation through the head, which have for their 

 object the prevention of this prodigious quantity of blood from pro- 

 ducing any injurious effects upon the tender cerebral substance, whether 

 by its pressure or by its unequal distribution, in consequence of its 

 stagnating in the vessels, or of its being too violently propelled against 

 them. Many conjectures have been formed respecting the object of 

 furnishing this organ with such an extraordinary quantity of blood, 

 but nothing is really known of the use to which it in applied, though 

 it may be admitted, to give a degree of plausibility to the opinion, that 

 the brain has some analogy to a secreting organ. Without doubt one 

 use both of the ventricles and the convolutions is to afford a more 

 extended surface by which the blood-vessels may enter the cerebral 

 substance at a greater number of points, and consequently in small 

 quantity at any one point, while at the same time they are more firmly 

 supported in their passage by the greater quantity of investing mem 

 brarie with which they are supplied. [Nuiivocs SYSTKM.] 



The brain of the vertebrate animals differs considerably from that 

 of man, and more in proportion to their low position in the scale ol 

 development. The most obvious distinction between the brain ol 

 man and that of the other mammalia ia its diminished size in mosl 

 of the latter. The moment the skull-cap is raised, the difference 

 between the full rounded appearance of the former and the com 

 pressed flattened shape of the latter cannot fail to be observed. The 

 convexity of the middle lobes is strangely lessened, and the posterior 

 lobe is in a manner lost in quadrupeds. If the brain is now removec 

 from the cranial cavity, the difference in bulk between that of man 

 and the inferior animals is strikingly displayed. The brain of the ox 

 scarcely weighs a pound : the average weight of the brain of the 

 human being is more than 2J pounds. 



In man the brain is supposed to constitute about l-35th part o 

 the weight of his body. In the dog, averaging the different breeds 

 it is 1-1 20th part; in the horse it is only the 450th part, in the shee] 

 tli.- V50th part, and in the ox the 800th part. 



As an illustration of the greater si/e and development of the nerves 

 of sense in animals, the olfactory one may be selected. In man, whi 

 has other means of judging of the qualities of his food, and of sur 



ounding objects, than by the sense of smell, the olfactory nerve is not 

 ne-fourth the size of that of the horse ; in the ox, which is not so 

 much domesticated as the horse, and oftener sent into the field to 

 hift for itself, it is considerably larger ; it is larger still in the 

 wine, which has to search for a portion of its food buried in the earth, 

 >T deeply immersed in refuse or filth ; and it is largest of all in the 

 log, whose acuteness of scent renders him so useful a servant to man. 

 The different development of the medulla oblongata in different 

 mimals may be adduced as another proof of the admirable adaptation 

 of each to the situation which it occupies and the functions which it 

 discharges. The medulla oblongata is the prolongation and conden- 

 sation of the medullary matter of the brain, and it is the origin of 

 ;hat portion of the spinal cord which is devoted to organic life. In 

 the human being, the breadth of it is only a seventh part of that of 

 ,he brain ; in the horse and the ox, it is nearly a third ; and in the 

 dog it is more than a half. In every part of the brain of the 

 quadruped the medullary portion preponderates, and the ciueritious 

 s deficient. 



In the smaller quadrupeds the comparative size of the brain 

 ipproaches nearer to that of the human being. In the mouse it is a 

 43rd part of the weight of the animal. 



The brain of the larger birds agrees with that of the mammalia in 

 the smallness of its bulk, compared with the development of the same 

 organ in the human being. The brain of the eagle is not more than 

 a 260th part of the weight of the bird. The brain of the goose is not 

 more than a 360th part. If in some of the lesser birds, as in the 

 chaffinch and the redbreast, it approaches to the proportionate size of 

 .hat of the human being, it is, as in the smaller quadrupeds, on account 

 of the quantity of medullary matter required for the origins of the 

 nerves ; and the cineritious matter forms only a very small part of the 

 brain. The brain of the bird has no convolutions on its surface ; no cor- 

 pora striata in the ventricles ; no pons varolii between the brain and the 

 spinal cord ; and the origins of the optic nerves are separate from the 

 brain, and lie behind and below it. 



In fishes the brain is yet more diminished in proportionate size. 

 In some species it does not constitute a 2000th part of the bulk of 

 the fish. It scarcely half fills the cranial cavity, but is surrounded 

 by a cellular tissue containing a transparent semifluid mass. It 

 singularly varies in different species. It consists of at least four or 

 more rounded eminences, placed in pairs opposite to each other and 

 forming two parallel lines ; and there is often only a very slight con- 

 nection between these lines, or the eminences of which either of them 

 is composed. The two principal hemispheres of the brain and the 

 optic thalami are always present. The olfactory nerves often form a 

 third pair of tubercles anterior to these and the cerebellum, and are 

 always found posteriorly on the mesian line. The optic nerves usually 

 cross each other without any intermingling of medullary matter. The 

 cineritious substance is found in an exceedingly small proportion in 

 the brain of fishes. 



There is no brain properly speaking in the Inrertebrota. In the worm, 

 the upper ganglion of the nervous system, which represents the brain, 

 is placed near to, or may be said to be perforated by, the superior por- 

 tion of the oesophagus, and thence proceed little white threads or cords, 

 which run along the course of the digestive canal. In insects the 

 upper ganglion usually surrounds the ccsophagus, and a ganglionic 

 system of nerves can generally be traced proceeding from it. In the 

 larvae of insects the brain is inclosed in a horny cavity. The spinal 

 cord proceeding from it pursues its course through the whole of the 

 abdomen, presenting evident ganglia at different points, from which 

 nerves are distributed ; while from the intermediate spaces are given 

 out other nerves without ganglia, presenting a rude but satisfactory 

 sketch of the combined systems of sensitive and motor nerves dis- 

 covered by modern physiologists. 



(Quain, Elementi of Anatomy; Grant, Outlines of Comparative 

 Anatomy ; Carpenter, Principle* of Physiology.) 



BRAIZE, a Fish. [PAGRUS.] 



BRAKES. [PTEBIS.] 



BRAKES ROCK, the vulgar name for iheAllosorus crisptte, a plant 

 belonging to the natural order Pvlypodiaccce. Attotorus is known by its 

 nearly circular sori, which are at length confluent, and are concealed 

 by the reflexed margin of the frond. A. criiput has a slender very 

 brittle stem, which attains a height of from 6 to 1 2 inches. It grows 

 in stony places on mountains throughout Great Britain. 



BRA'MA, a genus of Fishes of the order Acanthopten/gli and family 

 Squamipfnnfs. Dorsal, anal, and ventral fins more or less scaly ; body 

 much compressed, somewhat ovate when viewed laterally ; the head 

 rather obtusely terminated ; mouth when shut almost vertical ; teeth 

 slender, placed both in the jaws and palatines ; branchiostegous rays 

 seven. But one species of this genus is known, Brama Rail. M. Cuvier 

 mentions the Mediterranean as the chief locality for this fish, but at 

 the same time he says that it occasionally wanders into the ocean. 

 It appears however that it is not so local as M. Cuvier supposes, 

 numerous specimens having been found on different parts of our own 

 coasts. 



11,-nma Rail measures from about 1 to 2 feet in length; it is of a 

 deep blue colour, becoming silvery towards the belly. The dorsal fin 

 has 34 rays and the anal 30 rays. The tail is large and forked ; pectoral 

 fins rather long and narrow ; ventral fins small : the scales extend as 



