but they all come within the normal range of variation. There can be no doubt that this range is considerable. There would appear to be not only individual, but also racial, differences; there is a remarkable passage in Vatsyayana's _Kama Sutra_ describing the varying behavior of the women of different races in India under the stress of sexual excitement--Dravidian women with difficulty attaining erethism, women of the Punjaub fond of being caressed with the tongue, women of Oude with impetuous desire and profuse flow of mucus, etc.--and it is highly probable, Ploss and Bartels remark, that these characterizations are founded on exact observations.[108] The various phenomena included in Roubaud's description of the condition during coitus may all be directly or indirectly reduced to two groups: the first circulatory and respiratory, the second motor. It is necessary to consider both these aspects of the process of detumescence in somewhat greater detail, although while it is most convenient to discuss them separately, it must be borne in mind that they are not really separable; the circulatory phenomena are in large measure a by-product of the involuntary motor process. With the approach of detumescence the respiration becomes shallow, rapid, and to some extent arrested. This characteristic of the breathing during sexual excitement is well recognized; so that in, for instance, the _Arabian Nights_, it is commonly noted of women when gazing at beautiful youths whose love they desired, that they ceased breathing.[109] It may be added that exactly the same tendency to superficial and arrested respiration takes place whenever there is any intense mental concentration, as in severe intellectual work.[110]