164 



ON THE EXTERNAL SENSES 



and cautious research, gave it as a part of their report, that the doctrine of the 

 origin and action of the nerves is probably correct; but that this doctrine 

 does not appear to have any immediate or necessary connexion with that part 

 of Dr. Gall's theory which relates to distinct functions possessed by distmct 

 parts of the brain.* The origin, and distribution, and action, however, of the 

 nervous trunks have since been far more accurately traced out by Mr. 

 Charles Bell, M. Magendie, and various other physiologists ; while, in refuta- 

 tion of the doctrine that ascribes distinct functions to distinct parts of the 

 brain, it may be sufficient to observe, for the present, that many of the nerves 

 productive of different functions originate in the same part, while others, pro- 

 ductive of the same function, originate in different parts. 



There is no animal whose brain is a precise counterpart to that of man ; and 

 it has hence been conceived, that by attending to the distinctions between the 

 human brain and that of other animals, we might be able to account for their 

 different degrees of intelligence. But the varieties are so numerous, and the 

 parts which are deficient in one animal are found connected with such new 

 combinations, modifications, and deficiencies in others, that it is impossible 

 for us to avail ourselves of any such diversities. Aristotle endeavoured to 

 establish a distinction by laying it down as a maxim that man has the largest 

 brain of all animals in proportion to the size of his body ; a maxim which has 

 been almost universally received from his own time to the present period. 

 But it has of late years, and upon a more extensive cultivation of compara- 

 tive anatomy, been found to fail in various instances : for while the brain of 

 several species of the ape kind bears as large a proportion to the body as that 

 of man, the brain of several kinds of birds bears a proportion still larger. 

 M. Sommering has carried the comparison through a great diversity of genera 

 and species : but the following brief table will be sufficient for the present 

 purpose. The weight of the brain to that of the body forms — 



In man, from to 3*3 part. 



— several tribes of simia — 



— dog - - - Tir — 



— elephant - - — 



— sparrow - - 2V — 



— canary bird - - tV — 



— goose - - — 



— turtle (smallest) - -y^^^ — 



M. Sommering has hence endeavoured to correct the rule of Aristotle by a 

 modification, under which it appears to hold universally ; andj thus corrected, 

 it runs as follows : " Man has the largest brain of all animals in proportion to 

 the general mass of nerves that issue from it." 



Thus, the brain of the horse gives only half the weight of that of a man, 

 but the nerves it sends forth are ten times as bulky. The largest brain which 

 M. Sommering ever dissected in the horse-tribe weighed only lib. 4oz., 

 while the smallest he ever met with in an adult man was 2lb. Sjoz.f 



It is a singular circumstance, that in the small heart-shaped pulpy sub- 

 stance of the human brain, denominated the pineal gland, and which Des 

 Cartes regarded as the seat of the soul, a collection of sandy matter should 

 invariably be found after the first few years of existence ; and it is still more 

 singular, that such matter has rarely, if ever, been detected but in the brain 

 of a few bisulcated animals, as that of the fallow-deer, in which it has been 

 found by Sommering ;J and that of the goat, in which it has been traced by 

 Malacarne.§ 



The nervous system of all the vertebral or first four classes of animals,— 

 mammals, birds, amphibials, and fishes, — are characterized by the two follow- 

 ing properties : — first, the organ of sense consists of a gland or ganglion with 



*For an examination of the general subject of craniology and physiognomy, see Series iii. Lecture xiii. 

 t Study of Med. iv. 11, 2d edit. 



X Dissertatio de hasi Encephali, 1778, and Tabula basf os Encephali, 1799. See Blumenb. p. 292. 

 j Dissert, p. 10. See also Blumenbach, Anat, Comp. $ 206. 



