I. Nature of the brain, its ramifica CLASS IV. 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. substitutes. Sömmering 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 tions and The comparison fails in various cases. Aristotle's maxim corrected by Söm mering: weight of the brain to that of the body, forms and thus corrected, holds universally. Rule applies to animals in the general de scent of the scale of ani mal life. Turtle (smallest)....5688 M. Sömmering has hence endeavoured to correct the rule of Aristotle by a modification under which it appears to hold universally; and, 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 a 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. Sömmering ever dissected in the horse kind, weighed only 1lb. 4oz. while the smallest he has met with in an adult man was 2lb. 5 oz. But the remark applies farther than to man: for this acute physiologist has been able to trace a direct proportion between the degree of intelligence in every class of animals, and the bulk of the brain, where the latter bears an inverse proportion to the nerves that arise from it. And we may hence observe, in passing, as indeed we have already hinted, that the nerves seem rather to be a product of the brain than the brain of the nerves: for it is much more easy to conceive how a fountain may become exhausted in proportion to the magnitude of its * Diss. de basi Encephali. Götting. 1778. 4to. streams, than how a reservoir can be augmented in proportion to the minuteness of its channels. CLASS IV. I. Nature of the ramifications and character Upon a general survey, I may observe that the nervous brain, its structure of all vertebral animals, comprising the first four classes of the Linnéan classification, mammals, birds, substitutes. amphibials, and fishes, is characterized by the two follow- Distinctive ing properties. Firstly, the organ of sense consists of a of the nergland with a long chord or spinal marrow descending from it; and, secondly, that both are securely inclosed in vertebral a bony case or covering. In man, as I have already observed, this gland is (with a few exceptions) larger than in any other animal in proportion to the size of the body; and, without any exception whatever, in proportion to the size of its dependent column. 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 does not much exceed the diameter of the spinal marrow itself. vous struc ture of animals. In what varies as respect the scale descends. structure in Possess a It is not, therefore, to be wondered at that animals of a Nervous still lower description and without a vertebral column, should exhibit proofs of a nervous chord or spinal mar- animals. row without a gland or brain of any kind at the top; 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 either of a cranium or a spine; and in none of which we are able to trace more than a slight enlargement of the superior part of the nervous chord, or spinal marrow, as it is called, in animals possessing a spine; often consisting of one, and sometimes portionally larger than of two ganglions designed, apparently, to correspond with invertebral the organ of a brain; the descending column chiefly and enrichtaking the course of the esophagus and surrounding it. The nervous chord, however, in these animals is proportionally larger than in those of a superior rank; and, though sometimes simple, as in molluscous worms, in nervous chord pro animals, ed with ganglions. I. Nature of the brain, its ramitications and substitutes. CLASS IV. other cases, as in insects, is possessed at various distances of minuter ganglions or little knots, from which fresh ramifications of nerves shoot forth like branches from the trunk of a tree, and which may perhaps be regarded as so many distinct cerebels or little brains: having a close resemblance to the subordinate system of the intercostal nerve in man, as we have already traced it in its various ramifications and connexions. Ganglions probably minute cerebels. Whether a nervous structure in zoophytes and infusory animals? liar make. In worms of apparently the simplest make, as zoophytes and infusory animals, no distinct structure can be discerned, and particularly nothing like a nervous system. The hydra or nearly transparent polypus found so frequently in the stagnant waters of our own country, with a body of an inch long, and arms or tentacles in proporTheir pecu- tion, scems, when examined by the largest magnifying glasses, to consist of a congeries of granular globules or molecules, not unlike boiled sago surrounded by a gelatinous substance; in some tribes solitary, in others catenated. And hence, whatever degree of sensation or voluntary motion exists in such animals can only be conceived as issuing from these molecules acting the part of nervous ganglions detached, or connected. And on this account M. Virey has elegantly divided all animals into three classes according to the nature of their nervous configuraration; as first, animals with two nervous systems, a cerebral and sympathetic, including mammals and birds, amphibials and fishes. Secondly, animals with a sympathetic nervous system alone, surrounding the esophagus, as mollusca and shell-fishes, insects and proper worms. And, thirdly, animals with nervous molecules, as echini, polypes, and infusory animalcules, corals, madrepores, and sponges; all which in M. Virey's classification are included under the term zoophytes. Virey's classification of animals from their supposed difference of nervous structure. Touch the only sense common to hence sup posed by The only sense which seems common to animals, and which pervades almost the whole surface of their bodies, all animals: is that of general touch or feeling; whence M. Cuvier, supposes that the material of touch is the sensorial power in its simplest and uncompounded state; and that the other senses are only modifications of this material, though pe Cuvier to be the base of the other senses. CLASS IV. I. Nature of the ramifications and culiarly elaborated by peculiar organs, which are also capable of receiving more delicate impressions*. Touch however, has its peculiar local organ, as well as the other brain, its senses, for particular purposes, and purposes in which unusual delicacy and precision are required; in man this peculiar power of touch is well known to be seated in the nervous papillæ of the tongue, lips, and extremities of the fingers. Its situation in other animals I shall advert to presently. substitutes. Touch, though dif fused generally, has its local organs as other The differences in the external senses of the different well as the orders and kinds of animals consist in their number and degree of energy. when situated in man. Vertebral animals the same num ternal All the classes of vertebral animals possess the same number of senses as man. Sight is wanting in zoophytes, in various kinds of molluscous, and articulated worms, and in the larves of several species of insects. Hearing ber of exdoes not exist, or at least has not been traced to exist, in many molluscous worms and several insects in a perfect state. Taste and smell, like the general and simple sense of touch, seem seldom to be wanting in any in the animal. The local sense of TOUCH, however, or that which is of a more elaborate character and capable of being exercised in a higher degree, appears to be confined to the three classes of mammals, birds, and insects and even in the last two it is by no means common to all of them, and less so among insects than among birds. senses as man. Number diminishes lower classes. Local sense of touch confined to mammals. Exists in organs in different different tribes. In the racoon In quadru In apes and macaucoes, constituting the quadrumana of Blumenbach, it resides partly in the tongue, and tips of the fingers as in man, but equally, and in some species even in a superior degree, in their toes. (ursus lotor) it exists chiefly in the under surface of the peds. front toes. In the horse, and cattle orders, it is supposed by most naturalists to exist conjointly in the tongue, and snout, and in the pig and mole to be confined to the snout alone; this however is uncertain; as it is also, though there seems to be more reason for such a belief, that in * Anatom. Comparat. 1. 25. CLASS IV. the elephant it is seated in the proboscis. Some physio I. Nature of the brain, its ramifica tions and logists have supposed the bristly hairs of the tiger, lion, and cat, to be an organ of the same kind; but there seems little ground for such an opinion. In the opossum substitutes. (and especially the Cayenne opossum) it exists very visibly in the tail; and M. Cuvier suspects that it has a similar existence in all the prehensil-tailed mammals. Blumenbach supposes the same sense to have a place in the same organ in the platypus or ornithorhyncus as he calls it, that most extraordinary duck-billed quadruped which has lately been discovered in Australasia, and, by its intermixture of organs, confounds the different classes of animals and sets all natural arrangement at defiance. The local organ of touch or feeling in ducks and geese and some other genera of birds appears to be situated in the integument which covers the extremity of the mandibles, and especially the upper mandibie, with which apparatus they are well known to feel for their food in the midst of mud in which they can neither see nor perhaps smell it. We do not know that amphibials, fishes, or worms possess any thing like a local sense of touch; it has been suspected in some of these and especially in the arms of the cuttle-fish, and in the tentacles of worms that possess this organ, but at present it is suspicion and nothing more. In the insect tribes, we have much reason for believing such a sense to reside in the antennas or in the tentacles; whence the former of these are denominated by the German naturalists fühlhorner or feeling-horns. This belief has not been fully established, but it is highly plausible from the general possession of the one or the other of these organs by the insect tribes, the general purpose to which they apply them, and the necessity which there seems for some such organ from the crustaceous or horny texture of their external coat. The senses of TASTE and SMELL in animals bear a very near affinity to the local sense of touch; and it is difficult to determine whether the upper mandible of the duck tribe, with which they distinguish food in the mud, may |