270 OF MUSCULAR CONTRACTILITY. 



be watching the same visual phenomena, and timing their observations by 

 the same clock ; for it has been remarked, that some persons see the same 

 phenomenon, a third or even half of a second earlier than others. There is no 

 reason to suppose from this, however, that there is any difference in the rate 

 of transmission of the sensory impressions in the two nerves. The fact seems 

 rather to be, that the sensorium does not readily perceive two different impres- 

 sions with equal distinctness ; and that, when several impressions are made 

 on the nerves at the same time, the mind takes cognizance of one only, or 

 perceives them in succession. When, therefore, both sight and hearing are 

 directed simultaneously to one object, the communication of the impression 

 through one sense will necessarily precede that made by the other. The 

 interval between the' two sensations is greater in some persons than in others ; 

 for some can receive and be conscious of many impressions, seemingly at the 

 same moment ; whilst in others a perceptible space must elapse. 



365. Amongst other important offices of the power of Hearing, is that of 

 supplying the sensations by which the Voice is regulated. It is well known 

 that those who are born entirely deaf, are also dumb, that is, destitute of the 

 power of forming articulate sounds ; even though not the least defect exist in 

 their organs of voice. Hence it appears that the vocal muscles can only be 

 guided in their actijn by the sensations received through the Ears, in the 

 same manner as other muscles are guided by the sensations received through 

 themselves ( 399). On this point, more will be said hereafter ( 413). 



CHAPTER V. 



OF MUSCULAR CONTRACTILITY. 



I. Of Contractility in general. 



366. THE Nervous System has no power of occasioning movement in any 

 part of the body, save by exciting to contraction certain structures, to which 

 the term Muscular is given. That one tissue should possess within itself the 

 property of contractility on the application of a stimulus, is no more wonderful 

 than that another should be capable of conveying sensory or motor influences, 

 or another of separating a peculiar secretion from the blood. Such contrac- 

 tile tissues are found in Vegetables as well as in Animals ; and instances of 

 their operation have been already referred to ( 13). The only essential 

 difference between the contractility* of Muscular Fibre, and that of the cells 

 of the Sensitive Plant, is that the former can be excited by the stimulus of 

 innervation, as well as by those of a physical or chemical nature, which will act 

 upon the latter. Muscular structure, as heretofore remarked, is employed in 



* The peculiar operation of this property in Muscular Fibre, the ordinary contraction 

 of which alternates with relaxation, has occasioned the distinctive term Irritability to be 

 applied to it. This term has been employed, however, in so many different 'senses 

 (being, by some Physiologists, used almost synonymously with the more general one of 

 Vitality), that it seems desirable to avoid adopting it for such a purpose. 



