BRAIN AND NERVOUS SYSTEM. 355 



performance of the other operations, ( 70S, p. 307,) or per- 

 haps it must suspend them altogether. 



816. If any of the involuntary operations, which require 

 the direction of the brain, but not the consciousness of the 

 mind, becomes difficult, and requires mental effort, the brain 

 can do little more than attend to it. Thus we maintain res- 

 piration and carry on the other operations in conjunction 

 with it ; but in paroxysms of asthma, or in croup, when the 

 whole nervous power is concentrated in the effort to breathe, 

 all labor of body, and all thought must be suspended, and 

 the whole nervous energy devoted to respiration. 



817. An accomplished musician can play several parts of 

 a tune with his fingers upon the organ, and read and sing the 

 words of the song, or call the figures of the cotillon ; but if 

 the tune is riot familiar, if it requires a special effort of the 

 attention to read the notes, he can neither sing the words of 



,the song nor call the figures of the dance. 



818. If the brain is occupied by any other efforts, or by 

 any disagreeable sensations, it cannot give its full attention 

 to any mental operation. When a man wishes to give the 

 undivided energy of his mind to any subject, he places him- 

 self in such a position that his whole frame is most easy. If 

 he sits, he selects a chair suited to the form of his frame, so 

 that his body and limbs are supported without effort ; and 

 regulates the temperature of the room, so that he is neither 

 hot nor cold; and the light, so that it. Is neither painfully 

 glaring nor insufficient for the easiest perception. Some 

 prefer a standing posture, and others will walk their rooms: 

 but most prefer the sitting position while they are in intense 

 thought ; but, in either arrangement, nothing external calls 

 the brain from the subjects of study. 



819. It is the fault of many school-rooms, that they are so 

 constructed that a very considerable portion of the attention 

 of the pupils is taken up with their uncomfortable physical 

 sensations, and with their endeavors to obtain relief. Instead 

 of the comparatively easy chairs, somewhat adapted to their 

 forms, which children are accustomed to enjoy at home, these 



