724 



CRANIUM. 



which it is itself exercised. The immediate 

 effect of frequently repeated stimulation of a 

 voluntary muscle, whether by physical or mental 

 stimuli, in a living or newly killed animal, is 

 gradual diminution or ultimate extinction, or 

 what is usually called Exhaustion of its Irri- 

 tability ; which is gradually restored when the 

 stimulation is discontinued and the muscle is 

 at rest. 



But the theoretical conclusions which have 

 been drawn from this fact have greatly exceeded 

 the legitimate inferences. It is by no means 

 clear that such increased action of involuntary 

 muscles, as results from causes of the kinds 

 just mentioned, which exalt or increase their 

 contractile power, is necessarily followed by 

 any corresponding depression. On the con- 

 trary, in the case of violent exercise, in many 

 instances of mental agitation and excitement, 

 and in the course of certain febrile and inflam- 

 matory diseases, we see the heart's action 

 greatly and permanently increased, without 

 evidence of any subsequent loss of power which 

 can reasonably be ascribed merely to the cir- 

 cumstance of increased action. 



It is true that the effect of many stimulating 

 substances, such as alcohol, is first to excite, 

 and after atime to weaken or depress, the actions 

 of the heart and circulating system ; but as we 

 know that an equal or greater degree of excite- 

 ment from exercise, from exciting passions of 

 mind, or from inflammatory disease, may exist 

 without producing any such subsequent de- 

 pression, we ought to regard the loss of power 

 which follows the excessive use of such sub- 

 stances, as an ulterior effect of these substances 

 themselves, rather than as the result of the mere 

 circumstance of previous increased action.* 

 Although, therefore, we consider all exertions 

 of the irritability of muscles as necessarily im- 

 plying intervals of relaxation, and are aware of 

 the exhaustion of irritability by excessive sti- 

 mulation, yet we do not see that the operation 

 of those agents which augment the vital power, 

 particularly of the involuntary muscles, is ne- 

 cessarily followed by a corresponding loss of 

 power. 



Further, it has been often alleged that the 

 vital power of Irritability is not only expended 

 or exhausted by excessive action, but. likewise 

 increased or accumulated by rest. But there is 

 no evidence whatever that rest does more 

 than merely restore the power that had been 

 lost by previous exertion. A muscle or set of 

 muscles which has been weakened by excessive 

 excitement, and regained its power by rest, 

 may remain quiescent for an indefinite time 

 thereafter, and will not only not continue to 

 gain power, but will gradually lose, after a 

 time, that which it had previously possessed. 

 The idea of the accumulation of Irritability by 

 long-continued inaction has been thought to be 

 supported by the fact, that the stimulating 

 effect of Heat on all vital action, is greatest 

 when it is applied after long-continued Cold. 

 But this seems manifestly to be owing to the 



* See Gregory's Conspectus, art. De Remediis 

 Stimulant! bus. 



principle that the stimulating effect of heat on 

 vital action is proportioned, not merely to the 

 temperature that may be applied, but chiefly 

 to the degree of change of temperature under- 

 gone in a given time; of which point many 

 illustrations might be given, and which neces- 

 sarily implies that the effect of Heat must be 

 much increased by its I eing applied after Cold. 

 Another law, which may be deduced from 

 observation of repeated exertion of living con- 

 tractile parts, is of great importance both in 

 physiology and pathology; viz. that the ulti- 

 mate effect of such repeated exertion, with 

 sufficient intervals of repose, is to augment both 

 the hulk and strength of muscular fibres, and 

 facilitate the subsequent excitation of vital 

 action, whether in voluntary or involuntary 

 muscles. This is seen in the state of hyper- 

 trophy of the muscular fibres of the arms of 

 labourers, of the legs of dancers, of the heart, 

 in those who have disease of the valves of the 

 aorta, of the bladder, in those who have 

 disease of the prostate gland or stricture of the 

 urethra ; and is in fact only a part of a more 

 general law, that the habitual exertion (within 

 limits consistent with health) of all vital 

 powers, is naturally attended with an increased 

 flow of blood to the organs exerting those 

 powers, and with an increase of their nutrition. 

 And the counterpart of this is seen in the very 

 slow and gradual, but ultimately extreme dimi- 

 nution, not only of the vital properties, but of 

 the bulk and characteristic appearance, of mus- 

 cular parts which have been, from any cause, 

 kept very long in a state of absolute inaction. 

 According to the observation of Andral, the 

 structure of muscles may in these circum- 

 stances be so altered, that they become ulti- 

 mately hardly distinguishable from cellular 

 texture. The act of Nutrition, and therefore the 

 organization of muscular fibres, as well as of 

 other living parts, is manifestly intended by 

 nature to be, in a certain degree, dependent on 

 the exertion of their own vital power ; and one 

 effect of that exercise of vital power is to 

 solicit or attract the living fluid to the part 

 concerned in it, in a manner which the re- 

 searches of physiologists have not yet satisfac- 

 torily elucidated. 



( W. P. Alison.) 



CRANIUM (in anatomy) Gr. tt^anov ; Fr. 

 Crane; Germ. H irnschadel ; Ital. Cranio. 



The cranium is the protective investment of 

 the brain, on which it is moulded, and the 

 form of which, in warm-blooded animals, it 

 represents. It also incloses and protects the 

 organ of hearing. 



In cold-blooded animals there is not this 

 adjustment of the surfaces of the brain and its 

 .case; but, although in them the parietes of the 

 cranium are expanded beyond the limits of the 

 brain,* the principle of formation is neverthe- 



* Thus, according to Desmoulins the area of a 

 vertical section of the brain in the European tor- 

 toise is nearly one-third less than the area of the 

 cranial cavity ; and in Fishes, whether osseous or 

 cartilaginous, the disproportion is constantly still 

 greater. ED. 



