NERVOUS SYSTEM OF VERTEBRATA. 121 



part by the par vagum ; and though the lateral cords of the sympathetic may 

 be traced, they are almost destitute of ganglia. Even in the highest Verte- 

 brata, some of the glands, of which the secretion is most directly influenced 

 by the condition of the mind, are supplied with most of their nerves from the 

 cerebro-spinal system ; thus, the lachrymal and sublingual glands receive large 

 branches from the fifth pair, and the mammary glands from the intercostal 

 nerves. It may therefore be regarded as not improbable, that the organic 

 fibres contained in these nerves, and principally derived from the ganglia at 

 their roots, are the most direct channels through which the processes of nutri- 

 tion and secretion are influenced by mental emotions ; and that the office of 

 the distinct visceral system, is rather to bring these functional changes into har- 

 mony with each other than to supply any condition necessary for the separate 

 operations themselves. 



161. The Spinal Cord, with its encephalic continuation the Medulla 

 Oblongata, may be regarded as constituting the essential part of the nervous 

 system of Vertebrata. Although the Cerebral Hemispheres in Man bear so 

 large a proportion to it in size, that the Spinal Cord seems but a mere append- 

 age to them, the case is reversed when we look at the other extremity of the 

 scale ; the Cerebral Hemispheres jn many Fishes being but ganglionic pro- 

 tuberances from the Medulla Oblongata. Moreover, the fact that animals are 

 capable of living without the brain, whilst they at once die if deprived of the 

 spinal cord, sufficiently demonstrates this. The spinal cord, then, when 

 viewed in relation to the nervous system of the Invertebrata, may be regarded 

 as including their respiratory, stomato-gastric, and pedal ganglia. That these 

 should be associated together, can scarcely be considered remarkable. It 

 is obviously convenient that they should all be enclosed in the bony sheath 

 provided for their protection; and their closer relation favours that sympathy 

 of action, which is so important in animals of such complex structure and 

 mutually dependent functions as the higher Vertebrata. An animal, either 

 congenitally or experimentally deprived of its cerebral hemispheres, is very 

 much in the condition of one of the Acephalous Mollusca. It can perform 

 those respiratory movements on which depend the maintenance of its circula- 

 tion, and consequently its whole organic life ; it can swallow food brought 

 within its reach, and it can, in some degree, exert its locomotive powers to 

 obtain it ; but it is unconscious of the direction in which these can be best 

 employed, and is dependent upon the supplies of food that come within its 

 grasp. The Acephalous Mollusca are so organized, that they find support 

 from the particles brought in by their respiratory current ; but the more highly- 

 organized Vertebrata are not capable of so existing, and they must have the*ir 

 food provided for them by an exertion of the mental powers. So long as an 

 encephalous Vertebrate animal is duly supplied with its requisite food, so long 

 may it continue to exist ; and thus it is seen that the operations of the brain 

 are rather connected with the intelligence than with the blind, undesigning 

 instinct of the animal. 



162. It is only in the Vertebrata, that the difference between the afferent 

 and efferent fibres of the nerves has been satisfactorily determined. The 

 merit of this, discovery is almost entirely due to Sir C. Bell. He was led to 

 it by a chain of reasoning of a highly philosophical character; and though 

 his first experiments on the spinal nerves were not satisfactory, he virtually 

 determined the respective functions of their two roots, by experiments and 

 pathological observations upon the cranial nerves, before any other physio- 

 logist came into the field.* Subsequently his general views were confirmed 

 by the very decided experiments of Mtiller; but, until very recently, some ob- 



* See British and Foreign Medical Review, vol. ix. p. 140, &c. 



