60 COMPARATIVE ANATOMY 



both dorsally and ventrally by a median longitudinal narrow 

 cleft or fissure the dorsal and ventral fissures, but neither 

 of these extends to its most posterior part. Nor is the cord 

 of the same diameter throughout, but it is swollen in the 

 regions of the second and third, and again in the regions of 

 the fifth and sixth vertebrae to form the brachial and sacral 

 enlargements. Posteriorly to the seventh vertebra the cord 

 thins out rapidly, and is continued into the urostyle as a 

 slender thread, the filum terminale. 



Ten pairs of nerves are given off from the spinal cord to the 

 trunk and limbs. The first of these, which marks the com- 

 mencement of the spinal cord (for there is no other demarca- 

 tion between it and the hindmost part of the brain), passes out 

 by the inter-vertebral foramen between the first and second 

 vertebrae. It courses round the side of the throat, coming 

 near to the surface just behind the angle of the lower jaw, and 

 there it turns forward and runs to the root of the tongue. 

 This nerve is known as the hypoglossal. The second spinal 

 nerve is large ; it leaves the neural canal in the foramen 

 between the second and third vertebrae, and passes straight 

 outward, supplying branches to the shoulder-girdle and arm ; 

 hence it is called the brachial nerve. It is joined more or 

 less completely, at a little distance from its exit from the verte- 

 bral column, by the third spinal nerve. The two nerves supply 

 a number of small branches beyond the point where they unite; 

 and these branches, inosculating with one another, form a net- 

 work known as the brachial plexus. The fourth, fifth, and 

 sixth spinal nerves run obliquely backward from the inter- 

 vertebral foramina proper to them, and supply the muscles of 

 the trunk. The seventh, eighth, ninth, and tenth spinal nerves 

 (the last-named passing put of the foramen of the urostyle) 

 run obliquely backward to the region of the pubis, and there 

 the seventh and eighth unite to form a single trunk, which is 

 joined a little farther back by the ninth, and, a little farther 

 back still, by a branch of the tenth. The large nerve so 

 formed is the sciatic, and the network formed by the union 

 of the nerves which are combined in it is called the sciatic 

 plexus.* As the spinal cord narrows rather suddenly in the 

 region of the sixth nerve, the roots of the seventh, eighth, 



* The details of the sciatic plexus are subject to some variation, but the 

 arrangement described here may be considered normal, 



