

Eeceptoes, Neurons, and Effectoks 119 



Chemical substances carried by the blood to muscles are also an im- 

 portant factor in the maintenance of tone. An increase, for example, in 

 the quantity of adrenalin, secreted into the blood from the supra-renal 

 glands, heightens the general muscular tone. This effect is not considered, 

 however, as a direct tonic effect, but as a sensitizing of the muscle, so that 

 the nerve currents produce relatively greater effect. 



Smooth muscle, whether in the intestines, blood vessels, skin, or else- 

 where is in general subject to the same laws of stimulation as is striped 

 muscle. It seems, however, to be excited by other than nerve action, and 

 in particular, contraction of certain fibers may stimulate adjacent fibers. 

 There remains, however, some uncertainty on this point, as also on the 

 points concerning the causation of the rhythmic contraction and relaxation 

 of cardiac muscle. 



The acceleration and inhibition of glandular activity is brought about 

 by nervous activity both directly and indirectly. Directly, the nerve cur- 

 rents conveyed to the gland cells seem to stimulate them to greater activity, 

 or to check their activity. 25 Indirectly, currents to the muscular coats of 

 the arteries supplying the glands, by dilating and contracting the arteries 

 and therefore increasing or decreasing the blood supply, increases or de- 

 creases the glandular activity, since the material for the secretion is de- 

 rived from the blood. 



The important bodily activity, in short, is muscle and gland action, and 

 I this is to a large extent directed by the nervous system. The nervous sys- 

 tem, on the other hand, is controlled by physical and chemical stimuli 

 from objects external to it. Efferent currents control the effectors, but 

 efferent currents are but the sequelae of afferent currents from the var- 

 ious receptors. The receptors, in turn, are excited to function by the 

 action of: 1. Pressure on the receptor (certain receptors in skin, mucous 

 membrane, and other tissues). 2. Light (retinal receptors). 3. Sound 

 (cochlear ending). 4. Substances in solution (taste bud receptor and re- 

 ceptors in the alimentary canal). 5. Gaseous substances (olfactory cells). 

 6. Temperature changes (receptors undiscovered). 7. Muscular contrac- 

 tion (muscle spindle receptors and receptors in smooth muscle). 28 The 

 last form of stimulation is not identical with pressure, since the receptors 

 in smooth muscle cannot be stimulated by pressing upon, pinching, or 



25 Contraction of the muscle fibers in the ducts of a gland may play an important 

 part in the pouring-out of the secretion. Thus, when an animal smells or sees food 

 the saliva appears, through contraction of the ducts, before there is a significant in- 

 crease in the secretion. 



26 This list of stimuli is probably not exhaustive. Vibration and electric current 

 may have normal and specific actions. 



