556 OF NUTRITION. 



which cannot with any probability be referred to an original excess in the supply 

 of blood, the enlargement of the arteries leading towards such parts being almost 

 certainly consequent upon their unusually rapid growth, just as in the case of 

 the uterine and mammary arteries of the pregnant female. The most remark- 

 able instances of the acquirement of increased formative activity, are presented 

 to us in that augmented growth of the nervous and muscular tissues, which is 

 consequent upon the exercise of their functional powers. This may be con- 

 sidered as to a certain extent a normal adjustment of the supply to the demand; 

 but there are some instances in which it takes place to such an extent as to 

 become a positive disease. Thus it not unfrequently happens, that if young 

 persons who naturally show precocity of intellect, are encouraged rather than 

 checked in the use of the brain, the increased nutrition of the organ (which 

 grows faster than its bony case) occasions pressure upon its vessels, it becomes 

 indurated and inactive, and fatuity and coma may supervene. Now although 

 in such cases there must probably have been some congenital tendency to pre- 

 ternatural activity of the brain, which manifests itself in precocity of intellect, 

 yet there is no doubt that this may be augmented by the " forcing system' 7 of 

 education ; whilst, on the other hand, it may be controlled by a system of 

 management adapted to the peculiar circumstances of the case. Excess of 

 muscular development is peculiarly prone to show itself in the involuntary 

 muscles ; but this production is in almost every instance the result of the 

 demand for increased muscular exertion, which is consequent upon some obstruc- 

 tion to the usual function of the part. Thus an extraordinary hypertrophy of 

 the muscular coat of the urinary bladder is often seen as a consequence of ob- 

 struction to the exit of the urine, through the presence of a stone in the bladder 

 or of a stricture in the urethra ; so, again, hypertrophy of the muscular coat of 

 the gall-bladder may take place as a consequence of obstruction of its duct by a 

 gall-stone ; hypertrophy of the muscular coat of any part of the alimentary canal 

 may take place in consequence of stricture lower down ; and even hypertrophy 

 of the heart is generally, if not always, attributable to obstruction to the exit of 

 the blood which it propels, resulting either from stagnation of the pulmonary 

 circulation by the deficient aeration consequent upon disease of the lungs (in 

 which case the hypertrophy is limited to the right side of the heart), or from 

 thickening or induration of the semilunar valves or narrowing of the orifices of 

 the aorta and pulmonary artery. It is curious, moreover, to observe that hyper- 

 trophy of muscles frequently becomes a source of increased nutrition of the 

 bones to which they are attached ; this being manifested, not merely in the 

 augmented bulk of the bones of limbs that are specially exercised, but also in 

 the increased prominence of the ridges and processes to which the muscles are 

 attached. This adaptiveness on the part of the formative activity of the osseous 

 tissue, is curiously manifested also in the relation of the skull to the brain ; 

 for if the bulk of the brain be not too rapidly augmented, the skull will enlarge 

 accordingly, and this (in some instances) not merely by the extension of its 

 normal bones, but by the intercalation of new osseous elements, the " ossa 

 wormiana '," whilst, on the other hand, if there be a diminution in the bulk of 

 the brain, the cranium may adapt itself to this also, by a thickening on its in* 

 ternal surface, or concentric hypertrophy this change, rather than a diminu- 

 tion in the whole substance of the skull, being more liable to take place in cases 

 in which the cranial sutures have already closed, and the nutrition of the bone 

 has become inactive, so that the modelling process, which consists in the ab- 

 sorption of old and the deposition of new osseous tissue ( 130), cannot take 

 place. 



597. The production of Tumors must be considered as a manifestation of an 

 excess of formative activity in individual parts, and as constituting, therefore, 

 a species of Hypertrophy. For a tumor may be composed of the tissues which 



