72 



AGE. 



bellum, is observed only at the top of the 

 animal scale, we might expect that in the pro- 

 gress of age there would be a change in the 

 relations of these parts to the whole mass, but 

 we cannot find that any researches have been 

 prosecuted in elucidation of these points. 



Most of what we have predicated of the pro- 

 gressive actions in the brain, is likewise appli- 

 cable to the cerebellum and spinal marrow, 

 and nerves. The latter parts, however, are 

 more forward in their organization, being de- 

 voted to the more primitive functions of sensa- 

 tion and voluntary motion, while the former is 

 the instrument of the faculties more eminently 

 intellectual. The proportion of the cerebellum 

 to the brain at birth, is, according to Meckel, 

 as 1-23; the former weighing nearly 3| drachms, 

 the latter 9 or 10 oz. A month after birth the 

 ratio is 1*17 ; after six months, 1-8. 



The proportion between the spinal marrow 

 and the brain at birth, and for five months 

 after, is 1-107 or even 1-112 ; the brain at the 

 former period weighing 9oz. 4dr., and the 

 spinal marrow 45gr., while at the latter period 

 the cerebral organ weighs 21oz., and the spinal 

 l|dr. In the fretus of five months the propor- 

 tion is 1-63, of three months 1-18. In the 

 adult it is 1'40. The diminishing ratio of the 

 brain to the spinal marrow is in obvious har- 

 mony with the elongation of the vertebral 

 column, and with the general growth of the 

 members. The medulla oblongata is propor- 

 tionally larger in early than in advanced life ; 

 the corpora pyramidalia and olivaria being dis- 

 tinct and prominent; a fact which corresponds 

 with the development of the brain. 



The longitudinal dimensions of the corpora 

 quadrigemina at birth exceed those of the adult 

 period ; after the former period they increase 

 only in their transverse diameter. 



The concretions of the Pineal glands have not 

 begun to be formed till the seventh year. They 

 are sometimes wanting in very advanced age, 

 according to the observations of Meckel and 

 the Wenzels, which we have had oppor- 

 tunities of verifying by our own dissections. 

 The number of these bodies increases with the 

 progress of life, and their colour is paler in 

 youth and old age than in intermediate 

 periods.* 



So much then for the nervous organs of 

 sensation. Our attention must next be directed 

 to the mechanism intermediate to the nerves, 

 and the excitants of sensation. The simplest 

 kind of sensation is that which informs or re- 

 minds us that we are possessed of bodily parts, 

 such as members and internal organs. The me- 

 chanism employed, if there be any,js unknown. 

 Nerves are distributed through the tissues, we 

 feel those tissues, and conclude that these feel- 

 ings result from relations between the nerves and 

 the other textural molecules with which they are 

 in contact. These feelings must of course vary 

 with age because the tissues alter, but whether 

 the susceptibility is increased we cannot say, 

 and only venture to remark that the proba- 



* For further details sec the works of Meckel and 

 Tiedemann. 



bility of this being the case is suggested by the 

 fact, that adults are more subject to perver- 

 sions of sensibility than children; witness the 

 various nervous, hypochondriacal, and hys- 

 terical disorders with which adults are almost 

 exclusively visited. 



The next order of sensations in respect of 

 simplicity are those of tact, or those by which 

 we are made acquainted that foreign bodies 

 are in contact with our skin. It is perhaps in 

 some respects only a modification of the first- 

 mentioned sensation, but it requires the pre- 

 sence of something not belonging to us. It is 

 true that other parts than the skin may convey 

 the notion of an external body being applied to 

 them, but they do not afford any perception of 

 the qualities of the body ; it is merely the 

 affection of themselves which is produced by 

 that body. We are aware that all sensation 

 may be analysed in the same manner with 

 similar results, but it is enough for our present 

 purpose that the sensation excited on the skin is 

 less selfish, if we may use the term in this sense, 

 and ought to be so, in order that it may serve 

 its office of supplying some knowledge of the 

 external world. Doubtless the organization of 

 the epidermis and of the skin itself, as well as 

 the greater distribution of nervous matter, occa- 

 sion the difference. The dermoidal tissue in 

 modifying the external cause stands in the same 

 relation to the nerves of tact, as the eye to the 

 optic nerve, or the nose to the olfactory. The 

 organ of tact is affected by age; the skin in 

 very early life appears less susceptible of im- 

 pressions, and differs in its tissue, the papillae 

 being less developed. A change, however, is 

 soon effected in this respect, and as we advance 

 towards manhood, it becomes less gelatinous 

 and more fibrous. It must be confessed, how- 

 ever, that the modifications which it undergoes 

 in reference to its function of sensation, are not 

 well defined. This circumstance is owing to 

 the variety of sensations to which it ministers, 

 such as (in addition to what we have men- 

 tioned) feelings of heat and cold, dryness 

 and moisture, &c. and, secondly, to its being 

 also an organ for other and very different func- 

 tions, such as transpiration, secretion, and ab- 

 sorption. 



Touch has a far more complicated mechanism 

 than tact. It is one of the senses properly so 

 called, or the special senses, and like the others 

 of its class is distinguished by its requiring the 

 assistance of muscles. Its sensations are com- 

 pounded of tact and muscular resistance, and 

 the organ is that wonderful instrument the hand. 

 The imperfect state of this organ in infants must 

 have been noticed by every one ; it is generally 

 closed and capable of grasping but very feebly ; 

 at all events a long time occurs before the 

 little being learns to arrange the sensitive 

 tips of the fingers, and to adjust the thumb in 

 such a manner as to ascertain with nicety the 

 form, consistence, and other properties of 

 bodies. Whether the skin is less sensitive in 

 these subjects we cannot say, but it is quite 

 certain that the muscles, which effect the digital 

 motions alluded to, are not developed any more 

 than those in other parts of the body. P'ine- 



