1908.) ——- Innervation of Antagonistic Muscles. 561 
termed by him “crossed reflex III.” The “shortening reaction” in the knee- 
extensor of the spinal dog is in my experience also readily evoked. To 
evoke it, the animal may be held up under the shoulders with the hind hmbs 
hanging freely and the knee then gently raised by lifting it from under the 
lower end of that joint. This somewhat flexes the hip and extends the knee. 
A strong tonic contraction of the knee-extensor at once sets in and is usually 
long maintained. This reaction is frequently though not regularly accom- 
panied by a movement of flexion at the crossed knee. 
There is another crossed reflex, well seen in the spinal dog, which bears 
resemblance to the two just mentioned. A movement of extension of one hip 
causes a movement of flexion at the crossed hip. To evoke this all that is. 
necessary when the dog is supported as described above is to allow one of the 
thighs to drop into extension. The opposite hip flexes almost immediately 
the extension of the other commences. 
V. The reflex tonus of these skeletal muscles of the cat and dog is thus 
seen in these observations to be adjusted by a regulation which conforms, at 
least in regard to the results it ensures, with features observed by v. Uexkiill 
in the tonus of the musculature of Echinus, Ophioglypha, and Sipunculus. 
v. Uexkiill showed that in those cases stretching the muscle lessens the 
tonus init. “Stretching brings about a fall in the tonus of the muscle; the 
stretched muscle has a lower level of tonus than the unstretched.* When 
an unstriped muscle is stretched it follows the stretch readily. When the 
stretch is removed the muscle does not quickly return to its previous length. 
It exists, therefore, after removal of the stretch, in a different state of 
excitation from before the stretch. For although it once more is loaded only 
with its own weight, it is nevertheless longer than before.”t v. Uexkiill 
assumes, if I follow him rightly, that a shortened condition of the muscle 
evidences greater tonus, a lengthened condition less tonus. That may be so, 
but at present I hesitate to assume it. Apart from this, which seems a 
difference rather of interpretation than of fact, this description quoted here 
from v. Uexkiill applies fully to the case of. the vasto-crureus in decerebrate 
rigidity. And v. Uexkiill’s observations on this feature of the tonus of 
muscles in Kchinus, etc., have been found by other experimenters to hold 
good in the unstriped muscles of various other Invertebrata and in the 
unstriped muscles of various viscera in Vertebrates.~| With this feature of 
behaviour of the tonus in unstriped muscle that of the tonus of the extensor. 
muscles in the spinal dog and in the decerebrate cat conforms. 
a5 Ergebnisse d. Physiol.,’ part 2, 1904, and ‘ Zeitschr. f. Biolog.,’ vols. 44 and 46. 
+ Ibid. 
t Cf. Griitzner, ‘ Ergebn. d. Physiol.,’ part 2, 1904. 
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