A.NATOMY IN A NUTSHELL. 35 



the arm or brachium. On each side of the spinal cord are five plexuses, making 



five pairs in all. We learn the anatomy of the lateral half of the body and the 

 other half is similar, with a few exceptions in the viscera. The first plexus is 

 made of the anterior divisions of the first four cervical nerves and is called the 

 Cervical Plexus. The next plexus in order is called the Brachial, and is com- 

 posed of the anterior branches of the fifth, sixth, seventh and eighth cervical 

 nerves and part of the first dorsal nerve. In the dorsal or thoracic region there 

 are no plexuses, except sympathetic. There are plexuses of nerves where there 

 is much work to be done by the nerves. The next plexus is the Lumbar which 

 is composed of the anterior branches of the first, second, third and part of the 

 fourth lumbar nerves. The part of the fourth lumbar nerve which does not 

 enter into the formation of this plexus joins the fifth lumbar nerve and these 

 two nerves make the lumbo-sacral cord which, although it has lumbar nerves in 

 it, belongs to the sacral plexus. This cord with the anterior divisions of the 

 first, second, third and part of the fourth sacral nerves make the Sacral Plexus. 

 The remaining part of the fourth nerve with the fifth and coccygeal nerve, make 

 the Coccygeal Plexus. As the spinal cord is so much shorter than the spinal 

 canal, the nerves leave the cord above the foramina where they leave the spinal 

 canal. 



There are eight cervical nerves although only seven cervical vertebra?, as 

 the first cervical, or suboccipital nerve, is above the atlas and the eighth cervi- 

 cal nerve below the seventh cervical vertebra, or vertebra prominens. The 

 eight cervical nerves leave the cord above the spinous process of the sixth cer- 

 vical vertebra. 



There are twelve pairs of dorsal or thoracic nerves; the first six of these 

 lea 1 ,!' the cord between the spinous processes of the sixth cervical and the fourth 

 dorsal vertebra; the lower six dorsal or thoracic nerves leave the cord between 

 the spinous processes of the fourth and eleventh dorsal. The five Lumbar 

 nerves leave the cord between the spinous processes of the eleventh and twelfth 

 dorsal. The five sacral nerves leave the cord between the spinous processes of 

 the twelfth dorsal and the first lumbar. The lower nerve roots have such a 

 long course between the cord and the foramina that they are called cauda equina 

 (horse tail.) 



LESSON VII. 



Clavicle (Key.)— This is a long bone but it has no medullary canal as (he 

 other long bones have. It is quite elastic, and with the scapula acts ;is a cush- 

 ion to lessen the force of blows upon the shoulder. In sonic animals it does not 

 articulate with any other bone, but is held in position by the muscles. This 

 bone and the scapula make the shoulder girdle. It is present in those animals 

 which have claws and use the upper or front extremity for other purposes than 

 that of progression. It acts as a fulcrum for t he various movements of t he upper 

 extremity. It is situated at the upper anterior part of the chest, between the 

 manubrium of the sternum and the acromion process of the scapula. It has a 

 double curve with the convexity anterior at the sternal extremity and the con- 



