A text-book of physiology for medical students and physicians . sensory and motor paths are concerned in the act. Ac- * See Eckhard, Geschichte der Entwickelung der Lehre von den Reflex-erscheinungen, Beitriige zur Anatomie u. Physiologic, Giessen, 1881, 142 REFLEX ACTIONS. 143 cording to the neuron theory, therefore, the simplest reflex arcmust consist of two neurons: the sensory neuron, whose cellbody lies in the sensory ganglia of the posterior roots or ofthe cranial nerves, and a motor neuron, whose nerve cell liesin the anterior horn of gray matter of the cord or in the motornucleu
A text-book of physiology for medical students and physicians . sensory and motor paths are concerned in the act. Ac- * See Eckhard, Geschichte der Entwickelung der Lehre von den Reflex-erscheinungen, Beitriige zur Anatomie u. Physiologic, Giessen, 1881, 142 REFLEX ACTIONS. 143 cording to the neuron theory, therefore, the simplest reflex arcmust consist of two neurons: the sensory neuron, whose cellbody lies in the sensory ganglia of the posterior roots or ofthe cranial nerves, and a motor neuron, whose nerve cell liesin the anterior horn of gray matter of the cord or in the motornucleus of a cranial nerve. The reflex arc for the spinal cordis represented in Fig. 66. The arc may, however, be morecomplex. The sensory fibers entering through the posteriorroots may pass upward through the entire length of the cordto end in the medulla, and on the way give off a number ofcollaterals as is represented in Fig. 67, or they may makeconnections with intermediate cells which, in turn, are con-nected with one or more motor neurons (Fig. 68). According. Fig. 66.—Schema to show the connection between the neuron of the posterior root and thaneuron of the anterior root,—the reflex arc. to these schemata, one sensory fiber may establish reflex connectionswith a number of different motor fibers, or, a fact which must beborne in mind in studying some of the well-known reflex activitiesof the cord and medulla especially, a sensory fiber carrying animpulse which eventually reaches the cortex of the cerebrum andgives rise to a conscious sensation may, by means of its collaterals,connect with motor nuclei in the cord or medulla and thus at thesame time give origin to involuntary and even unconscious re-flexes. Painful stimulation of the skin, for example, may giveus a conscious sensation of pain and at the same time reflexlystimulate the vasomotor center and cause a constriction of thesmall arteries. The fact that in this case two distinct events occurdoes not necessitate the
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