PHYSIOLOGY OF MUSCULAR MOTION. 383 



from one bone to another ; by their rq^tance preserving the bones in 

 situ; and by their suppleness admitting the necessary motion. 



The interior of all these articulations is lubricated by a viscid fluid, 

 called synovia. This is secreted by a peculiar membrane of a serous 

 nature ; and its use is to diminish friction, and, aMhe same time, to 

 favour adhesion. The mode in which it is secreted, and its chief pro- 

 perties and uses, will be the subject of future inquiry. 



In certain of the movable articulations, fibro-cartilaginous substances, 

 frequently called inter articular cartilages, are found between the articu- 

 lar surfaces, and not adherent to either. These have been supposed to 

 form a kind of cushion, which, by yielding to pressure, and returning 

 upon themselves, may protect the joints to which they belong ; and, 

 accordingly, it is asserted, that they are met with in joints, which have 

 to sustain the greatest pressure ; but M. Magendie 1 properly remarks, 

 that they do not exist in the hip or ankle-joint, which have constantly to 

 support the strongest pressure. The use, which he suggests, is more 

 specious; that they may favour the extent of motion, and prevent 

 displacement. 



The stability of the joints is likewise aided by the manner in which 

 muscles or tendons pass over them. These are contained in an aponeu- 

 rotic sheath, to prevent their displacement ; and thus the whole limb 

 becomes well protected, and dislocation unfrequent, even in those joints, 

 as that of the shoulder, which, as regards their osseous arrangement, 

 ought to be very liable to displacement. 



It has been suggested by Weber, that the head of the thigh-bone is 

 retained in situ, not by the power of the muscles or ligaments, but by 

 the pressure of the surrounding atmosphere ; and Lauer, 2 who repeated 

 Weber's experiments under the directions of Fricke, of Hamburg, is of 

 opinion, that atmospheric pressure must be classed among the means by 

 which the lower extremity is kept in apposition with the trunk of the 

 body. 



PHYSIOLOGY OF MUSCULAR MOTION. 



By voluntary motion or that effected by the muscular system of ani- 

 mal life, we mean contraction of the muscles under the influence of 

 volition or will. This influence is propagated along the nerves to the 

 muscles, which are excited by it to contraction. The encepJialon, spi- 

 nal marrow, nerves, and 'muscles are, therefore, organs of voluntary 

 muscular contraction. 



Volition is a function of the encephalon, and might have been with 

 much propriety included under the physiology of the intellectual and 

 moral acts ; but as it is so intimately concerned with muscular motion, 

 it was judged advisable to defer its consideration. That in man and 

 the higher animals, it is a product of encephalic action is proved by 

 many facts. If the brain be injured in any manner; by fracture of the 

 skull, or by effusion of blood, producing apoplectic pressure ; or if it be 

 deprived of its functions by a strong dose of any narcotic substance ; 



1 Precis Elementaire, 2de edit.,i. 292, Paris, 1823. 



8 Zeitschrift fur die gesammte Medicin, Band ii. Heft 3. 



