49 6 TEXT-BOOK OF PHYSIOLOGY. 



volitional movements. The pigeon remains in a state of 

 profound stupor, though retaining the capability of executing 

 reflex or instinctive movements. It can temporarily be aroused 

 by loud noises, light placed before the eyes, pinching of the 

 toes, etc., but it soon relapses into a condition of quietude. 

 Coincident with the destruction of the cerebrum there occurs 

 a loss of memory, reason, and judgment, and the animal fails 

 to associate the impressions with any previous train of ideas. 

 The higher the animal in the scale of development, the more 

 striking is the loss of mentality after removal of the cerebrum. 

 4. Experimental interference with the blood-supply to the cerebrum 

 is followed by a diminished or complete cessation of its activities. 

 There is perhaps no organ of the body that is so directly depend- 

 ent upon its blood-supply for the continuance of its activities 

 as the cerebrum. The supply of blood is furnished by four large 

 blood-vessels: viz., the two carotid and the two vertebral arteries. 

 These vessels, after entering the cavity of the skull, give off 

 branches which unite to form the "circle of Willis." From this 

 circle, large branches are given off which enter the cerebrum 

 and distribute blood to all its parts. After passing through the 

 capillaries the blood, greatly altered in chemic composition, is 

 returned by large veins. The large volume of blood that is 

 present in the brain and the marked changes in composition 

 that it undergoes while passing through the brain indicate a 

 very active and complex metabolism in this organ. By means 

 of the anatomic arrangement of the blood-vessels at the base 

 of the brain, the blood-supply is equalized. It also explains 

 why, when one, or even two, of the four large vessels are oc- 

 cluded by pathologic deposits or surgical procedures, brain 

 activity continues, though perhaps diminished in degree. Occlu- 

 sion of all four vessels, however, is at once followed by a complete 

 abolition of all forms of cerebral activity. An experiment per- 

 formed by Brown-Sequard illustrates the dependence of cerebral 

 activity on the blood-supply. A dog was beheaded at the 

 junction of the neck and chest. After a period of ten minutes 

 all evidences of life had entirely ceased. Four tubes connected 

 with a reservoir of warm defibrinated blood were then connected 

 with the four arteries of the head. By means of a pumping 

 apparatus imitating the action of the heart the blood was driven 

 into and through the brain. After a few minutes cerebral 

 activity returned, as shown by contraction of the muscles of 

 the face and eyes. The character of the contractions were such 

 as to convey the idea that they were directed by the will. These 

 vital manifestations continued for a period of fifteen minutes, 

 when on the cessation of the artificial circulation they disap- 



