COMPARATIVE ANATOMY. 



ether; and it may be observed, that the 

 sound produced by some of the larger 

 birds is exactly similar to the notes that 

 proceed from a clarionet or hautboy in 

 the hands of an untutored musician. The 

 inferior glottis exactly corresponds to the 

 reed, and produces die tone or simple 

 sound. The superior larynx gives it ut- 

 terance, as the holes of the instrument ; 

 but the strength and body of the note de- 

 pend upon the extent and capacity of the 

 trachea, and the hardness and elasticity of 

 its parts. The convolution and bony cells 

 of the windpipe, therefore, may be com- 

 pared with the turns of a French horn, 

 and the divisions of a bassoon ; and they 

 produce the proper effects of these parts 

 in the voices of those birds in which they 

 are found. 



UKAIN AND NEKVES, AND ORGANS OF 

 SENSE. 



The parts subservient to the animal 

 functions, which, constituting the leading 

 character of animals, have derived their 

 name from that circumstance, afford to 

 our observation a more clear and manifest 

 gradation, from the most simple to the 

 most compound structure, than any others 

 in the animal economy. 



In some of the most simple animals of 

 the class vermes, particularly among what 

 are called zoophytes, little or no dis- 

 tinction of similar parts (or structures) 

 can be discerned, and we are unable to 

 recognize any thing as a particular ner- 

 vous system, or even as a part of such a 

 system. The power of sensation and vo- 

 luntary motion which these possess, as 

 well as any other order or class of the ani- 

 mal kingdom, proves that the nervous 

 matter must be uniformly spread through- 

 out their homogeneous substance. The 

 almost transparent polypes (hydra,) which 

 are often found with a body of an inch 

 in length, and arms or tentacula of a 

 proportionate size, appear to consist, 

 when surveyed in the best light by the 

 strongest magnifying powers, of nothing 

 but a granular structure connected into 

 a definite form by a gelatinous sub- 

 stance. 



In many other vermes, and in insects, 

 particular nerves can be distinguished, 

 arising in general from a chord running 

 the whole length of the body, and called 

 the spinal marrow, the superior extre- 

 mity of which part, slightly enlarged, 

 constitutes the brain. The latter organ, 

 however, in both classes of cold and red- 



VOL. III. 



blooded animals, and still more in those 

 which have warm blood, has a much more 

 complicated structure, and a far greater 

 relative magnitude : all animals are, how- 

 ever, exceeded in both these points by 

 the human subject, which, according to 

 the ingenious observation of the learnt- d 

 Sommerring, possesses by far the largest 

 brain, in proportion to the size of the 

 nerves which arise from it. 



The vast superiority of man over all 

 other animals in the faculties of the mind, 

 which may be truly considered as a ge- 

 neral distinction of the human subject, 

 led physiologists, at a very early period, 

 to seek for some corresponding difference 

 in the brains of man and animals. They 

 naturally investigated the subject, in the 

 first instance, by comparing the propor- 

 tion which the mass of the brain bears to 

 the whole body ; and the result of this 

 comparison in the more common and do- 

 mestic animals was so satisfactory, that 

 they prosecuted the inquiry no farther, 

 but laid down the general proposition, 

 which has been universally received since 

 the time of Aristotle, that man has the 

 largest brain in proportion to his body. 

 Some more modern physiologists, how- 

 ever, in following up this comparative 

 view in a greater number of animals, dis- 

 covered several exceptions to the general 

 position. They found that the propor- 

 tion of the brain to the body, in some 

 birds, exceeds that of man ; and that se- 

 veral mammalia (some quadrumana, and 

 some animals of the mouse kind) equal 

 the human subject in this respect. 



As these latter observations entirely 

 overturned the conclusion which had 

 been before generally admitted, Som- 

 merring has furnished us with another 

 point of comparison, that has hitherto 

 held good in every instance : viz. that of 

 the ratio, which the mass of the brain 

 bears to the nerves arising from it. 



Let us divide the brain into two parts; 

 that which is immediaely connected with 

 the sensorial extremities of the nerves, 

 which receives their impressions, and is 

 therefore devoted to the purposes of 

 animal existence. The second division 

 will include the rest of the brain, which 

 may be considered as connecting the 

 functions of the nerves with the facul- 

 ties of the mind. In proportion then as 

 any animal possesses a larger share of 

 the latter and more noble part ; that is, 

 in proportion as the organ of reflection 

 exceeds that of the external senses, may 

 we expect to find the powers of the 



S s 



