220 CLINICAL APPLIED ANATOMY. 



this disease. The type which is known as the progressive spinal 

 muscular atrophy, in which spastic symptoms are absent, cor- 

 responds to a degeneration of the lower motor segment or 

 neuron. This segment comprises an anterior cornual cell, and 

 its axis cylinder process, which is prolonged through the corre- 

 sponding anterior root to a muscle. In the other type of disease, 

 spastic symptoms coexist with muscular wasting. This type 

 is known as amyotrophic lateral sclerosis, and is characterised by 

 degeneration of the upper motor segment or neuron, as well as 

 the lower. The upper neuron comprises a pyramidal cell of the 

 motor cortex with its axis cylinder process, which is prolonged 

 through the internal capsule and pyramidal tracts to become 

 functionally connected, through the medium of the grey matter 

 of the anterior cornu, with the lower neuron. 



In both types the muscles which waste appear to be successively 

 attacked according to the order in which their cell groups are 

 arranged in the cord. The cervical enlargement which corre- 

 sponds to the brachial plexus is the most common site of 

 the disease, and it commences with equal frequency at 

 either extremity of this enlargement. Occasionally the upper 

 part of the lumbar enlargement suffers, and rarely other parts. 

 In its later stages the disease may spread upwards to the bulbar 

 nuclei and the signs of glosso-labio-laryngeal palsy be super- 

 added. 



The upper part of the cervical enlargement contains the nerve 

 cell groups of the shoulder muscles, and its lower part those 

 corresponding to the small muscles of the hand. Accordingly 

 the deltoid muscle or the muscles of the thenar eminence are 

 usually the first muscles in which the disease is recognised. 



When the muscles of the thenar eminence are involved the 

 power of abducting the thumb from the plane of the palm and 

 also that of opposing the thumb to the fingers, are first to 

 become defective, since these movements are almost entirely 

 performed by the short thumb muscles. At the same time 

 the thenar eminence becomes wasted. Ultimately the thumb 

 becomes rotated outwards so that its palmar surface looks 



