THE AUTONOMIC NERVOUS SYSTEM 201 



tern, the adrenals are stimulated to great activity, and pour out 

 their product into the blood stream. Thus at the time when in- 

 creased adrenin is advantageous to the organism it is provided. 

 The persistence of the bodily effects of strong emotion after the 

 emotion itself has subsided may be explained by the continued 

 presence of adrenin in the blood. 



The reinforcement of the thoracico-lumbar autonomic mechan- 

 ism is only one phase of the emergency function of adrenin. An- 

 other, and very interesting, feature of its action is in connection 

 with the fatigue of the neuro-muscular junctions described in an 

 earlier paragraph (p. 198). We saw there that the effect of fatigue 

 on these junctions is to make the passage of impulses over them 

 difficult. Recently the important discovery has been made that 

 adrenin has the property of counteracting this fatigue, and thus 

 making the muscles more accessible to nervous impulses. The 

 value of this property in time of emergency is obvious. It explains 

 a familiar fact that was unexplained before, namely, the "strength 

 of desperation." Why a man in a tight place should suddenly ex- 

 perience an access of strength we now know is because in connection 

 with the powerful emotions engendered by his situation there is an 

 outpouring of impulses over his thoracico-lumbar autonomic sys- 

 tem. His adrenal bodies are stimulated thereby 'to abundant 

 production of adrenin; the adrenin is carried by his blood to all his- 

 muscles, and there makes the access of nerve impulses to the 

 muscles more ready. The gain is not in -actual muscular strength, 

 but in ability to use to the full the strength already present. 



The Thyroid. This organ lies in the neck on the sides of the 

 windpipe and consists usually of a right and a left lobe united by a 

 narrow isthmus across the front of the air-tube. It is about thirty 

 grams (one ounce) in weight; in the disease known as goiter it is 

 greatly enlarged and its structure altered. The thyroid is dark red 

 in color and very vascular, richly supplied with nerves, and is 

 subdivided by connective tissue into cavities or alveoli, the largest 

 of which are just visible to the unaided eye. Each alveolus is lined 

 by a single layer of cuboidal cells, and filled by a glairy fluid known 

 as the thyroid colloid. 



From the gland can be obtained, in addition to the usual or- 

 ganic compounds, a peculiar substance containing a large percen- 

 tage of iodine, and known as iodothyrin. This compound was 



