40 -^n Introduction to Zoology. 



fications of an uniform type, and so admirably adapting every creation for 

 its place in nature,* without joining in the feeling so well expressed 

 by the poet, as quoted by Dr. Roget : — 



" In human works, though laboured on with pain, 

 A thousand movements scarce one purpose gain. 

 In God's one single can its end produce, 

 Yet serves to second too some other use." — Pope. 



We Lave thus considered the mechanism of the muscles, the prime 

 movers, and the apparatus on which they act : we have next to consider 

 the nervous system, the mysterious means which supply, if M'e may so 

 speak, information and impulse to all those muscles which are destined, 

 by motions ultimately dependant upon the perceptions and volition of the 

 animal, to maintain its requisite relations with external nature. It is 

 supposed indeed, that the contractions of the involuntary muscles, such as 

 the heart, whose constant actions are essential to the continuance of life 

 from moment to moment, and are carried on without our consciousness, are 

 independant of nervous influence ; but perhaps this point has not yet been 

 fully proved, for the muscles employed in respiration clearly are subject 

 to this power, and as you cannot injure the respiratory nerves without 

 affecting the circulation, who shall determine whether this is merely a 

 secondary or conjoint effect. The digestion of the stomach, and the effect 

 of the organs of secretion, equally involuntary and essential functions, have 

 been shewn to depend on nervous influence. (Very possibly, as has been 

 suggested, by their conveying some power analogous to that which pro- 

 duces the chemical decompositions eflected by galvanism.) But however 

 this may be, it is evident that all the voluntary muscles act only in virtue 

 of this nervous influence. If their nerves be intersected, they become pa- 

 ralysed. The nervous system, therefore, discharges a double office; first, 

 the nerves of sensation convey the impressions produced by external ob- 

 jects to the common sensorium ; then the nerves of motion transmit to the 

 limbs the orders, as it were, to execute the proper motions, which the 

 will, the high judicial power enthroned on that sensorium, has pronounced 

 to be necessary. 



* The order of birds presents the most beautiful examples of this admirable ac- 

 commodation of all parts of the s".:eleton, to a subserviency to the general end; 

 which here is flight through the air. As muscular forces, superior to any possessed 

 by mammalia are here requisite to impart adequate vigour to the motions of the 

 anterior or wing-bearing extremities, we here find the whole humero-sternal part so 

 modified, as to afford the most ample attachment. Levity being equally necessary, 

 the bones are throughout so constructed, as to combine a maximum of strength with 

 a minimum of weight, being universally hollow frames, of which the parietes are in 

 the most condensed state, and their cavities filled not with marrow but with air, 

 conveyed through large air cells communicating with the trachaea. The whole form 

 of the body is equally adapted for flight, and assuredly every sparrow manifests the 

 universal care and admirable skill of its contriver. 



