Retention and Loss of Hair. 409 



which blood can be cut off from a certain region. The 

 familiar phenomena of blushing and pallor show that the 

 nervous system, has a controlling influence over the size of 

 small arteries ; and the fact that the hair may become grey 

 in a few hours under violent emotion, carries with it the 

 lesson that in some way the nutrition of the hair is regu- 

 lated by this same nervous system. 



To understand the physiological bearings of this subject, 

 the somewhat complex relations of the blood-vessels of the 

 brain, the face, the bones and muscles of the head, and of 

 the scalp must be borne in mind. The arteries of the brain 

 find an outlet for their blood, when it has passed through 

 the capillaries and done its work, in those peculiar venous 

 channels lying on the inner tables of the skull known as 

 " sinuses" ; these communicate with the veins of the softer 

 osseous tissue (diploe) lying between the main tables of the 

 cranial bones, which again have connections with the veins 

 on the outside of the head. Now it is plain from this series 

 of connections, that pressure on the scalp must influence 

 the whole vascular system of the head back to the arteries 

 of the brain, unless in some way counteracted. Pressure 

 generally affects veins, from their superficial position, much 

 more than arteries. The bad effects of venous dilation are 

 seen in the slow-healing ulcers on the limbs of those with 

 dilated (varicose) veins. Throughout his paper Mr. Gouin- 

 lock has directed his attention almost wholly to arteries 

 rather than to veins. He has nowhere mentioned, what is 

 commonly enough seen by the physician, that anastomotic 

 arterial connections are especially opened up under the 

 exigencies of disease, as from the pressure of tumours, &c. 



Would nature refuse to combat the hard hat ? Could she not 

 adapt to it in a greater degree than Mr. Gouinlock's theory 

 supposes ? In looking at a plate portraying the course of 

 the arteries of the head, it will be noticed that the terminal 

 branches mount to the vertex of the skull and anastomose 

 with their fellows of the opposite side by very small off- 

 shoots. As it is the smaller branches of arteries that are the 

 most susceptible to changes in calibre, — can in fact be most 

 readily influenced by the nervous mechanism, it is easy 

 27 



