CUTANEOUS AND SOME OTHER SENSATIONS. 703 



corpus quadrigeminum and lateral corpus geniculatum. But the matter 

 needs further investigation. 



There remains the special sense of touch, but this we had better consider 

 in connection with sensations in general. 



ON THE DEVELOPMENT OF CUTANEOUS AND SOME OTHER SENSATIONS. 



590. The sensations with which we have just dealt arise through im- 

 pulses passing along special nerves or parts of special nerves, the optic nerve, 

 the olfactory nerve, etc. We have now to deal with sensations arising 

 through impulses along the nerves of the body generally. These are of 

 several kinds. In the first place, there are sensations which we may speak 

 of as " cutaneous sensations," the impulses giving rise to which are started 

 in the skin covering the body, or in the so-called mucous membrane lining 

 certain passages. These sensations, which, as we shall see in dealing with 

 the senses, are dependent on the existence of special terminal organs in or 

 near the skin, are sensations of "touch," in the narrow meaning of that 

 word, by which we appreciate contact with and pressure on the skin, and 

 the sensations of " temperature," which again we may, as we shall see, 

 divide into sensations of " heat " and sensations of " cold." These sensations 

 may be excited in varying degree by impulses passing along any nerve 

 branches of which are supplied to the skin. Then there are the sensations 

 constituting the " muscular sense," to which we have already referred, and 

 these again may be excited in any nerve having connections with the skeletal 

 muscles. 



As we shall see in dealing with the senses, when a nerve is laid bare and 

 its fibres are stimulated directly either by pressure, such as pinching, or by 

 heat, or by cold, or in other ways, the sensations which are caused do not 

 enable us to appreciate whether the stimulation is one of contact or pressure, 

 or of temperature, or of some other kind ; we only experience a " feeling," 

 which at all events when it reaches a certain intensity we speak of as " pain." 

 And we have reason to think that at least from time to time impulses along 

 various nerves gives rise to sensations which have been spoken of as those of 

 " general sensibility," by which in addition to other sensations, such as those 

 of touch and of the muscular sense, we become aware of changes in the con- 

 dition and circumstances of our body. When the stimulation of the skin 

 exceeds a certain limit of intensity, the sense of touch or temperature is lost 

 in, that is to say, is not appreciated as separate from, the sense of pain ; and 

 under abnormal circumstances acute sensations of pain are started by 

 changes in parts, for example tendons, the condition of which under normal 

 circumstances we are not conscious of appreciating through any distinct 

 sensation, though it may be that these parts do normally give rise to feeble 

 impulses contributing to " general sensibility." It may therefore be debated 

 whether " pain " is a phase of all sensations, or of general sensibility alone, 

 or a sensation sui generis. We shall have something further to say on this 

 matter when we treat of the senses ; meanwhile it will be convenient for 

 present purposes if we consider that the sensations we have to deal with just 

 now are the sensations of touch and of temperature, those of the muscular 

 sense, and those of general sensibility, including those of pain. 



591. The fairly convincing evidence that the occipital cortex has special 

 relations with vision, and the less clear evidence that other regions have 

 special relations with smell and hearing, suggest that special parts of the 

 cortex have special relations with the sensations now under consideration. 

 But in the cases of the senses of sight and smell we had a distinct anatomical 

 leading ; and we have seen how uncertain is the evidence where such an 



