396 MANUAL OF HISTOLOGY. 



vapour from the atmosphere ; and again, through the bulb, from the fluids 

 of the neighbourhood. It is upon this property that the interchange of 

 matters which takes place in hairs is dependent. The latter appears to 

 be by no means inconsiderable, as we may infer from the rapidity with 

 which hairs in some instances turn grey. The appearance of air within 

 the medulla follows upon a process of drying up which takes place there. 

 The shaft of the hair, however, is also saturated with the oil of the 

 sebaceous secretions. As Henle very properly remarks, we may recognise 

 the physiological condition of the skin from the state of the hairs ; their 

 brittleness on the one hand, and softness, pliancy, and glossy appearance 

 on the other. 



The growth and the nutrition of these structures takes place in a 

 manner exactly similar to that of the nails (p. 164). Multiplication of 

 cells takes place by segmentation at the lowest and softest part of the 

 bulb, kept up by supply of material through the blood-vessels of the 

 follicle, and more directly through those of the papilla?. And just as the 

 growth of nails can be accelerated by paring the free edge, so does cut- 

 ting of the ends of the structures in question favour tbeir rapid produc- 

 tion, as is seen in the beard after frequent shaving. On the other hand, 

 when both these tissues are left in the natural state, uncut, they seem 

 eventually to reach a point at which they cease to grow. We have 

 already seen that the nail may be completely reproduced so long as its 

 bed remains uninjured. The same is the case with the hair if its follicle 

 remain intact. This regeneration is called into play extensively during 

 the earlier periods of life ; and even later on, renewal takes place, to supply 

 the loss of large numbers of hairs which is sustained by the healthiest 

 body yearly, owing to disappearance of their roots. The hair destined 

 to be cast off is seen to be swollen at its lower end, and to be destitute of 

 the earlier excavation for the papilla. This is the "hair-knob" (Haar- 

 kolben) of Henle. Later on, loosening from the papilla, the whole hair 

 splits, and breaks up into a number of shreds, and becomes like a brush. 

 Pincus estimated the average daily loss of hairs from the heads of young 

 men to be, under normal conditions, from 38 to 108. 



The phenomena of growth observed accurately by Berthold in relation 

 to the nails have also been studied as regards the hairs. The latter grow 

 more rapidly at night than during the day, and in the warmer than in 

 the cold seasons of the year. They are also produced more quickly when 

 frequently cut. Thus the hairs of the beard when shaved every twelve 

 hours, show a growth in the year of 12"; when cut every twenty-four 

 hours, only 7 J" ; and when shaved every thirty-six hours, only 6f ". 



218. 



From the extensive researches of Valentin first, and then Koelliker, we 

 learn that the first rudiments of the hairs are formed in the human 

 embryo at the end of the third and beginning of the fourth month, appear- 

 ing first on the forehead and eyebrows (fig. 39.5). Here we find nodulated 

 or mamillated aggregations of cells (m) 0-0451 mm. in length, belonging 

 to the rete mucosum (b\ which sink gradually into the cutis by a process 

 of proliferation, pushing the adjacent part of the latter before them. 

 These cells increase rapidly in number, so that the collection soon becomes 

 larger and more flask-shaped. Around the latter there may now be 

 remarked a thin homogeneous transparent membrane (i), probably the 

 hyaline internal layer of the future follicle, about which the corium is 



