PHYSIOLOGY OF THE NERVOUS SYSTEM. 



722x 



tlie generative instinct -and the cerebellum, from 

 the occasional existence of an abnormal erection 

 of the penis, is not justified by the facts. This 

 symptom is far from being constant in cerebellar 

 disease indeed it occurs in but a very small 

 number of cases and, as a symptom pointing 

 to lesion of a particular portion of the cerebro- 

 spinal axis, it is much more indicative of 

 disease of the medulla oblongata or of the 

 cervical portion of the spinal cord. 



Office of the cerebral convolutions.' The 

 great sheet of vesicular matter which forms the 

 cortex of the human brain, is of such vast 

 extent that it is forced to assume the convo- 

 luted form in order to its being packed within 

 the ordinary compass of the cranium. A little 

 consideration will shew that the convoluted 

 form can be regarded no otherwise than as a 

 convenient mode of packing, and that the 

 number and depth of the convolutions are the 

 best indications of the superficial extent of this 

 expanse of vesicular matter. In certain cases 

 a slow and gradual accumulation of water 

 within the ventricles of the brain, causing a 

 corresponding enlargement of the cranium, 

 expands the matter of the cerebral hemispheres, 

 by which the ventricles are enclosed, and the 

 convolutions become unfolded. We thus ob- 

 tain a distinct demonstration of the true arrange- 

 ment of this part of the hemisphere, which 

 must be regarded as a nervous centre, con- 

 sisting of a vast mass of the potential vesi- 

 cular matter freely supplied with bloodvessels 

 from a vascular surface on its exterior (the pia 

 mater), and giving rise to an infinite multitude 

 of nerve-fibres, which pass from its internal 

 surface to the corpora striata and optic thalami, 

 the centres of volition and sensation. The 

 name which Mr. Solly has given to this ex- 

 panse of nervous matter, hemispherical gan- 

 glion, is very expressive, not only of its true 

 character as a centre of nervous power, but 

 likewise of the unity of the organ on each side, 

 consisting as it does of an uninterrupted layer 

 of vesicular matter with its emerging or im- 

 merging fibres, and not of a" great number of 

 different organs, as the term convolutions would 

 imply. 



This vesicular surface with the fibrous matter 

 which connects it with the optic thalami and 

 corpora striata forms by far the largest portion 

 of the encephalon in the higher classes of 

 animals. This fact alone ought to stamp it 

 with great physiological importance. But, 

 further, it is a well-proved fact, that the com- 

 plexity of the convolutions in the animal scale 

 is in the direct ratio with the advance of in- 

 telligence. At the same time it must be re- 

 membered that the complexity of the convo- 

 lutions is in part determined by the size of the 

 head and the capacity of the cranium. If, for 

 example, the habits and mode of life of the 

 animal require a small head and at the same 

 time a certain degree of intelligence, the brain 

 would exhibit a greater number and complica- 

 tion of convolutions than would be found in 

 an animal of corresponding intelligence, but 

 which required and possessed a larger head. 

 Hence neither the size nor the weight of the 



brain, whether absolute or in relation to the 

 body, affords any certain criterion of the extent 

 of the convoluted surface. Highly complicated 

 convolutions may exist along with a brain 

 both absolutely and relatively small. Thus, the 

 ferret, as shown by Leuret, whose habits require 

 a small head, has several well-marked convo- 

 lutions on each hemisphere, and a brain no 

 larger than that of the squirrel, which has no 

 convolutions at all, and which wants even the 

 few fissures which mark their first develope- 

 ment in the rabbit, the beaver, the agouti, &c. 

 And the last-named animals have the brain 

 both absolutely and relatively larger than that 

 of the cat, the pole-cat, the roussette, the 

 unau, the sloth, and the pangolin, all of which 

 possess convolutions. 



At the early periods of human life, in in- 

 fancy and childhood, the convolutions of the 

 brain are very imperfectly developed, but their 

 increase of size goes on simultaneously with 

 the advance of mental power. If the former 

 be arrested, or if some congenital fault pre- 

 vent the further growth of the convolutions, 

 the mental powers are of the lowest and fee- 

 blest kind, but little or not at all above those 

 of the brute with imperfect convolutions. In 

 all idiots the brain is not only small, but its 

 convoluted surface is extremely limited. 



Anatomy points to the conclusion that the 

 office of the convolutions is connected with the 

 functions of the mind. Perception, memory, 

 the power of abstraction, judgment, imagina- 

 tion, all possess, as instruments of corporeal 

 action, these folds of vesicular matter. And 

 it seems not improbable that the phrenological 

 view which assigns to certain convolutions a 

 special office connected with some particular 

 faculty or faculties is true. This is strongly 

 supported by the fact of a regular disposition 

 of certain primary convolutions in the various 

 classes of animals, so that each form of brain 

 has its proper convolutions, and that in tracing 

 the convolutions from the most simple to the 

 most complex, indications are found of the 

 persistence of the primary and fundamental 

 convolutions in the midst of many secondary 

 and superadded ones. 



It may be here mentioned that Gall was by 

 no means the first to assign this function to the 

 convolutions. Our countryman, Willis, in the 

 seventeenth century, distinctly advanced this 

 opinion, and conjectured that the various gy- 

 rations were intended for retaining the animal 

 spirits " for the various acts of imagination 

 and memory" within certain limits. 



It is important to ascertain the endowments 

 of the fibres which connect the vesicular sur- 

 face of the convolutions to the corpora striata 

 and optic thalami. They might be supposed to 

 possess similar endowments to those of sensi- 

 tive and motor nerves, if we adopted the views 

 of those who hold that all the nerves are con- 

 tinued up into the brain. This point, however, 

 has been settled in the most decisive manner by 

 experiments, dating as far back as the time of 

 Lorry.* Mechanical injury to them excites 



* Mem. dc 1'Acad. ilcs Sciences, 1760. 



