ORGANIC FORM AND ARCHITECTURE 693 



the impulse by an afferent fibre to subjacent ganglion-cells or motor 

 nerve-cells (M). Each of these gives off an efferent nerve-fibre to a 

 contractile element. Thus the chain has but two links— sensory 

 and motor nerve-cells — before it comes to the muscular cells, which 

 are often called the "effectors". Neuron is a convenient word for an 

 entire nerve-cell along with its bianches or fibres. 



From earthworms and their relatives onwards, there is often a 

 third link in the chain. The sensory neuron leads to an associative 

 (communicating or internuncial) neuron which is in turn linked to 

 a motor neuron. This makes a complete neuromuscular arc, as has 

 been already explained in the physiological section. If we use the 

 first letters of the terms, we have S (sensory), A (associative), M 

 (motor), and E (effector or muscular); S, A, M, E making a con- 

 venient mnemonic for those who are not familiar with these matters. 

 To a complete neuromuscular arc there is not in higher animals 

 any essential addition. The principle of the linkage is completed in 

 the earthworm's S, A, M, E. But it will be understood that instead 

 of a single sensory neuron at a strategic point, there is usually an 

 integrate of numerous cells making a sensory spot or organ. The 

 fibres from these cells may combine to form a sensory nerve, leading 

 to associative and motor neurons combined to form a ganglion. 

 The efferent fibres from the motor cells of the ganghon combine to 

 form a motor nerve which divides up into its fibres when it reaches 

 a muscle. 



On a muscle fibre there are not only the motor-endings of motor 

 fibres, which induce contraction, there are sensory nerve-endings 

 by which messages from the muscle are carried by sensory fibres to 

 sensory neurons. 



Ganglion cells are of very varied shape, but each consists of a 

 cell-body giving off branches. The cell-body contains a nucleus and 

 the cytoplasm shows characteristic granules and some evidence of 

 structural intricacy. One of the outgrowing processes of a motor 

 neuron is relatively long, branches very sparingly, and is known as 

 the axis-cylinder. In all but a few of the simplest Metazoa, this 

 nerve fibre is surrounded by a sheath called the neurilemma, said 

 to be formed by adjacent connective tissue; and in Vertebrate 

 animals each nerve fibre has in addition a medullary sheath. But 

 even in the higher Vertebrates there is an interesting persistence of 

 the Invertebrate type of non-meduUated or simply contoured fibre. 

 These occur in the sympathetic and olfactory nerves, and also in all 

 the nerves of the lamprey, the hag, and the lancelet, which are 

 primitive Vertebrate types. It is instructive to find this structural 

 detail confirming the derivation of Vertebrates from Invertebrates. 

 Besides the axis cylinder, the ganglion cell usually shows short pro- 

 cesses which ramify like the branches of a tree, and are called 

 "dendrites". They touch the dendrites of other neurons. 



