174 



ON THE EXTERNAL SENSES 



In man, from 2*2 tV P*'^' 



— several tribes of simia^'^ — 



— dog - - yiT — 



— elephant » - ^X- — 



— sparrow - 2V — 



— canary bird - — 



— goose - - 3^7 — 



— turtle (smallest) - jeVs — 



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

 a modification, under which it appears to hold universally ; and thus cor^ 

 rected, it runs as follows : " Man has the largest brain of all animals in pro- 

 portion 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 

 m. 6^oz.* 



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

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

 Descartes regarded as the seat of the soul, a collection of sandy matter 

 should mvariably 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 bisculated animals, as that of the fallow-deer, in 

 which it has been found by Sommering ;t 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 

 following properties : — first, the organ of sense consists of a gland or gan- 

 glion, with a long and bifid chord or spinal marrow descending from it, of 

 a smaller diameter than the gland itself ; and, secondly, both are severally 

 enclosed in a bony case or covering. 



In man, as we have already observed, this gland, or ganglion, is (with 

 a few exceptions) larger than in any other animal, in proportion to the 

 size of the body ; without any exception whatever in proportion to the 

 $ize of the chord or spinal marrow that issues from it. 



In other animals, even of the vertebral classes, or those immediately 

 before us, we meet with every variety of proportion ; from the ape, which, 

 ' in this respect, approaches nearest to that of man, to tortoises and fishes, 

 in which the brain or gangfion does not much exceed the diameter of the 

 spinal marrow itself 



It is not therefore to be wondered at that animals of a still lower de- 

 scription should exhibit proofs of a nervous chord or spinal marrow, with- 

 out a superior gland or brain of any kind ; and that this chord should 

 even be destitute of its common bony defence. And such is actually the 

 conformation of the nervous system in insects, and, for the most part, in 

 worms ; neither of which are possessed of a cranium or spme, and in none 

 of which we are able to trace more than a slight enlargement of the su- 

 perior part of the nervous chord or spinal marrow, as it is called in other 

 animals — a part situated near the mouth, and apparently intended to cor- 



♦ Stud. ofMed.iv. 11. 2d edit. 



t Dissertatio de basi Encephali, 1778 ; and Tabula baseos Encephali, 1799. See Bin- 

 menb. p. 292. 



I Dissert, p. 10. See also Blumebacb, Anat. Comp. § 206. » 



