598 THE FUNCTION OF THE SPINAL CORD 



intervals. Moreover, as these impulses are intended merely to produce 

 a tonic setting of the peripheral musculature, the aforesaid spinal 

 centers may be said to be tonically automatic, in contradistinction 

 to the bulbar centers, which may be considered as being rhythmically 

 automatic. 



It must be admitted, therefore, that the ganglion cells composing 

 these centers are in a state of constant tonic activity. This implies 

 that they produce ''subthreshold" impulses at regular intervals 

 which tend to retain the effector in a condition of functional alertness 

 ready at any time to yield maximal effects. Conversely, it may be 

 concluded that the loss of these impulses must diminish the tonus 

 of the effector and induce atrophic changes. You will have noticed 

 that the legs of a decerebrated frog, suspended from a hook, assume a 

 definite position of flexion, because the muscles are still in connection 

 with the motor cells of the cord. If one of the sciatic nerves is now 

 cut, the muscles on the side of the lesion immediately relax and allow 

 the leg as a whole to assume a more dependent position. In view of 

 the fact that these changes cannot be observed in a reflex frog after the 

 skin has been removed or after the posterior roots of the spinal cord 

 have been divided, it has been assumed that the tonic automaticity of 

 the spinal ganglion cells is due to a constant influx of subminimal 

 sensory impulses from the cutaneous receptors. In other words, it 

 is assumed that the ''inner stimulus" imparted to the motor cells of 

 the cord, finds its origin in sensory impulses of such slight intensity 

 that they cannot incite muscular contraction. Hence, the tonus 

 of a muscle is really a subminimal reflex phenomenon. The inherent 

 or inner stimulus upon which the automatic power of a nerve cell 

 depends, may thus be referred to subminimal sensory stimuli. 



Superficial, Deep, and Organic Reflexes. — In man, the spinal 

 cord aids in the production of a number of reflexes which possess a 

 very distinctive character and may on this account be employed for 

 purposes of diagnosis. Among the superficial or skin reflexes may be 

 mentioned the: 



(a) Cremasteric Reflex. — This reaction is elicited best by gently rubbing across 

 the inner aspect of the thigh. It consists in a raising of the scrotal sac and testicle 

 in consequence of the contraction of the muse, cremaster. 



(6) Scrotal Reflex. — It presents itself as a contraction of the tunica dartos in 

 consequence of an excitation applied to the skin of the scrotum. 



(c) Sternal and Abdominal Reflexes. — These reactions may be evoked by rapidly 

 drawing the blunt end of a rod-like instrument across the external surface of the 

 chest or abdominal wall. It consists in a contraction of the neighboring muscles. 



(d) Scapular Reflex. — It results in consequence of excitations of the skin in 

 the vicinity of the spinal column. The muse, rhomboidei contract. 



(e) Pharyngeal Reflex. — The touching of the posterior wall of the pharynx 

 incites a contraction of the muscles lining this passage. 



(/) Mammillary Reflex. — The stimulation of the integument in the vicinity of the 

 nipple is followed by an erection of the papilla. 



(g) Gluteal Reflex. — The muse, gluteus maximus contracts in consequence of 

 stimuli applied to the skin covering the buttocks. 



