520 



THE CELL AND MAMMALIAN HISTOLOGY 



unspecialised connective tissue and are called membrane bones. 

 Histologically there is no difference between the two, and the 

 cartilage of the former is dissolved before the bone is formed. 



Although cartilage is the chief skeletal material of the most 

 primitive existing fishes and precedes bone in ontogeny, bone is, 

 so far as is known, at least as old a tissue, for it makes most of 

 , the skeleton of the earliest 



motor 

 nerve 



^ — sensory fibre 



motor 

 tibrQ 



r)uclaus 



sensory ^'\.-ZrZt'E ^otor, 

 f/brQ-^^^'^^^^^<-J^^M tormina/ 



branches 



sensory 

 nerve 

 ending 



muscie 

 striations 



Fig. 405. — A piece of striped muscle 

 fibre with nerve endings upon it. — 

 From Thomson. 



known vertebrates, the ptera- 

 spids and cephalaspids of the 

 Silurian and Devonian periods. 

 The bony exoskeleton of these 

 is presumably represented by 

 the membrane bones of the head 

 and by the dentine of teeth 

 and the placoid scales of the 

 dogfish. Dentine has a structure 

 somewhat similar to that in 

 bone, though usually without 

 blood vessels, and in fishes 

 many transitional stages be- 

 tween the two are found. 



MUSCLE 



A motor nerve gives off motor nerve fibres which 

 lead to branched motor-endings— terminal 

 arborisation. 



A sensory nerve-ending is shown from which 

 impulses are carried by sensory fibres to a 

 sensory nerve. 



The remaining mechanical 

 tissue is muscle. There are three 

 sorts of this in vertebrates, 

 differing both structurally and 

 physiologically. We will take 

 first the muscle which is generally found attached to the bones. 

 From this fact it is called skeletal, from its embryological origin 

 it is called somatic, from its appearance striated or striped, and 

 from its physiology, voluntary. Any of these terms may be used, 

 but in zoology, as distinct from human anatomy, the last is 

 best avoided, as we know little of the will-power of a frog or a 

 fish or even of a mouse. The unit of striped muscle is the muscle 

 fibre, which is not a cell but a syncytium (Fig. 405). The length 

 of an average fibre is about 2.5 cms. and its diameter 0.05 mm., 

 but there are many both larger and smaller. On the outside is a 

 sheath, the sarcolemma, which encloses a liquid sarcoplasm 

 in which are a number of myofibrils and many nuclei. Each 



