154 Basic Structure of Vertebrates 



and motor tracts. Fibers extending from their region of origin toward 

 the brain are called "ascending"; extending in the reverse direction, 

 they are "descending" (Fig. 151). 



"Somatic" and "Visceral" 



Preliminary to a description of the nerves which are related to the 

 brain and spinal cord, it is necessary to make a digression for the pur- 

 pose of clarifying certain terms whose common usage is not intelligible 

 without explanation. 



The general structure of a vertebrate has been described (see p. 

 23) as "double-tubular" — one tube inside another. The outer tube is 

 the body-wall; the inner is the alimentary tube. There are important 

 structural differences between the two tubes. Muscle constitutes by far 

 the greater part of the substance of the body-wall, and such skeletal 

 parts as it may contain are necessary adjuncts of the muscle. Com- 

 pared to the body-wall, the alimentary tube contains a small amount 

 of muscle and a relatively large proportion of secretory tissue. The 

 segmented muscles which essentially constitute the body-wall — that is, 

 the muscles which effect the bodily movements and locomotion of the 

 animal — consist of highly specialized striated muscle-fibers (see p. 89). 

 These striated muscles develop from the paired dorsal mesodermal seg- 

 ments (somites) of the embryo (see p. 277). The muscle of the ali- 

 mentary tube is of the simple and more primitive nonstriated or 

 "smooth" variety and develops from ventrolateral unsegmented meso- 

 derm. In the coelomic space between alimentary tube and body- wall 

 are various organs, some of which, such as the lungs and liver, are 

 derivatives of the alimentary tube ; and others, such as the gonads and 

 urinogenital ducts, are derived from the body-wall. The kidneys are 

 products of the body-wall but usually they are not freely suspended in 

 the coelom. So far as these derivatives of the two primary tubes possess 

 muscle, it is, with a few exceptions, of the nonstriated variety. But the 

 muscles of the locomotor appendages, which are external derivatives 

 of the body-wall, are striated. 



In a strict anatomic sense, "somatic" refers to the body-wall; '* vis- 

 ceral" refers to the alimentary tube and all organs which occupy coe- 

 lomic space, or, more precisely (see p. 21), all organs which lie internal 

 to the somatic striated muscle. So far, then, it seems quite simple — 

 somatic muscle is segmented and striated, visceral muscle is unseg- 

 mented and nonstriated. But closer scrutiny of the body-wall reveals 

 complicating facts. The body-wall is richly penetrated by blood-ves- 

 sels. All except the very small blood-vessels, both arteries and veins, 

 possess layers of muscle in their walls, and this muscle is smooth. In 

 the walls of various glands developed in the skin, smooth muscle-fibers 



