928 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



disposition may be temporarily altered, all of which changes indicate that 

 the female organism at this time suffers a profound nervous shock. The loss 

 of the weighty function of reproduction and the adaptation to the new order 

 of events is not accomplished quietly. 



Senescence. The progressive diminution in the power of growth from 

 birth onward throughout life has been mentioned, and may be interpreted as 

 indicating that the process of senescence begins with the beginning of life. 1 

 In the broadest sense this is true, and is confirmed by a study of various 

 organic functions. In the more restricted sense senescence or old age com- 

 prises the period from about fifty years (in woman from the climacteric) 

 onward, during which there is a noticeable progressive waning of the vital 

 powers. The leading somatic changes accompanying old age are atrophic and 

 degenerative, but detailed statistics of this period are almost wholly wanting. 

 A marked cellular difference between the young and the old, which is shown 

 by nearly if not quite all tissues, is the relatively large nucleus and small 

 quantity of cytoplasm in the young, the proportions being reversed in the old. 

 This has recently been pointed out as follows by Hodge 2 in the nerve-cells of 

 the first cervical spinal ganglion : 



Pigment 

 little. 



33 per cent. 



Thus with the progress of age the nuclei become small and irregular in out- 

 line, and the cytoplasm pigmented, while the nucleoli are often wanting. The 

 nuclear differences are even more marked in the cerebral ganglia of bees, where, 

 moreover, aged individuals possess a smaller number of nerve-cells than the 

 young. They are in harmony with the growing belief in the function of the 

 nucleus as the formative centre of the cell. It has been shown that a decrease 

 in the weight of the whole brain occurs in both men and women, beginning in 

 the former at about fifty-five years, in the latter at about forty-five years. In 

 eminent men the decrease begins later. The thickness of the cortex and the 

 number of tangential fibres in it diminish especially after fifty years, and this 

 probably signifies a loss of cells. There is a decrease in general brain-power, 

 in power of origination, in the power to map out new paths of conduction and 

 association in the central nervous system and thus to form habits. Reaction- 

 time is lengthened. The delicacy of the sense-organs is noticeably less, and in 

 the eye the hardening of the crystalline lens and the weakening of the ciliary 

 muscle diminish the power of accommodation. The muscles atrophy and mus- 

 cular strength is reduced. The pineal gland, ligaments, tendons, cartilage, and 

 the walls of the arteries, show a tendency toward calcification, and the bones 

 become more brittle. Subcutaneous adipose tissue disappears, but a fatty de- 

 generation of cells is not uncommon, notably in all varieties of muscle-cells, 

 in nerve-cells, and probably in gland-cells. The pigment of the hairs disap- 



1 Cf. C. S. Minot: Journal of Physiology, xii., 1891. 



^C. F. Hodge: Anatomischer Anzeiger, ix., 1894; Journal of Physiology, xvii., 1894. 



