292 



PHYSIOLOGY 



of contractile tissue, innervated by a nerve network, and covered by a layer 

 of epithelium containing sense-cells. To this network is attached the 

 manubrium, which represents the mouth and stomach of the animal. In 

 such a mutilated jelly-fish it is easy to show that a stimulus applied to one 

 spot on the surface travels outwards from the excited spot to all parts of the 

 bell. The stimulus is propagated also to the manubrium, which in some 

 species bends in the direction of the excited spot that is to say, in the direc- 

 tion which represents the shortest possible path from the excited spot to the 

 manubrium. This preparation rarely presents any automatic activity. It 

 may react to a constant stimulus by a rhythmic series of contractions, but 

 remains perfectly motionless in the absence of stimulus. The unmutilated 



TIG. 136. Schema showing the utility of the multiplication of neurons and their 

 grouping in central ganglia. (CAJAL.) 



A, an ideal invertebrate with only cutaneous ' sensory ' neurons. 



B, invertebrate, such as a medusa, with sensory and motor neurons, but 

 no central nervous system. 



c, invertebrate (e.g. Annelid) in which the motor neurons are concentrated 

 in central ganglia. 



a, sensory neuron ; b, muscle ; c, motor neuron. 



jelly-fish presents rhythmic contractions of its sub-umbrella tissue which are 

 inaugurated in any or all of the marginal bodies and serve to drive the 

 animal onwards through the water in which it is immersed. The rhythmic 

 contractions may be initiated, augmented, or diminished, in response to 

 stimuli of light, mechanical irritation, or changes in the position of the whole 

 animal acting on the marginal bodies. In the reaction of an animal to ex- 

 ternal stimuli it must be an advantage if the energies of the whole can be 

 concentrated in defence of any one part and be evoked by a stimulus applied 

 at one point. Such a co-operation of the whole for the benefit of the part 

 involves the existence of direct paths from the stimulated point to all parts 

 of the animal if the reaction is to take place with any promptitude. In the 

 medusa we find a beginning of such ' long paths.' The general direction of 

 the fibres of the network is radial, and there is a concentration of such fibres 

 in the neighbourhood of the marginal bodies, so that an excitation can pass 

 more readily from a sense-organ to the mpnubrium than it can laterally along 

 the circumference of the animal. Moreover, a stimulus which is too slight 

 to excite a reflex contraction of the muscular tissue may travel along the 

 nerve tissue to each of the marginal ganglia and arouse th^se to a discharge of 

 motor impulses. We have therefore in the medusa sensory cells of different 

 sensibilities ; central cells specially adapted to reacting to and reinforcing a 



